cleanup: remove deprecated ClusterResourceSet feature gate references

[aman4433]

What this PR does / why we need it: Upstream Cluster API (CAPI) has permanently enabled the ClusterResourceSet feature and removed the associated feature gate in PR kubernetes-sigs/cluster-api#12950.

This PR cleans up our repository to align with this upstream change by removing "dead code" references:

• scripts/ci-e2e.sh: Removed the unused export EXP_CLUSTER_RESOURCE_SET=true.
• docs/book/src/developer/tilt.md: Removed instructions to set the feature gate in Tilt.
• templates/README.md & creating-a-cluster.md: Removed notes instructing users to manually enable this feature.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer: I performed an impact analysis to ensure no breaking changes were introduced:

• Go Code: Verified that we do not import the removed feature.ClusterResourceSet constant.
  grep -r "feature.ClusterResourceSet" . returned 0 results.
• Manifests: Verified that we are not passing the deprecated --feature-gates=ClusterResourceSet=true flag to the manager binary.
  grep -r "feature-gates" . confirmed the flag is not currently in use.

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Removed deprecated EXP_CLUSTER_RESOURCE_SET feature gate references from documentation and CI scripts.

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[Amulyam24]

@aman4433, it would be good to test a cluster creation with this change to be sure it works fine without any issues for the infra providers.

[aman4433]

I have tested my pr locally

Steps taken:

1. Created a local Kind management cluster.
2. Initialized it with the changes from this branch (clusterctl init).
3. Manually generated a workload cluster manifest using clusterctl generate ... --from ./templates/cluster-template.yaml without exporting the EXP_CLUSTER_RESOURCE_SET variable.

Result:
The ClusterResourceSet was successfully created, confirming that the feature gate is no longer required for generating these resources.

amanshrivastava@Amans-MacBook-Pro cluster-api-provider-ibmcloud % clusterctl generate cluster ${IBMVPC_NAME} \
  --kubernetes-version v1.33.0 \
  --target-namespace default \
  --control-plane-machine-count=1 \
  --worker-machine-count=1 \
  --from ./templates/cluster-template.yaml \
  | kubectl apply -f -
cluster.cluster.x-k8s.io/test-vpc-crs-removal created
ibmvpccluster.infrastructure.cluster.x-k8s.io/test-vpc-crs-removal created
kubeadmcontrolplane.controlplane.cluster.x-k8s.io/test-vpc-crs-removal-control-plane created
ibmvpcmachinetemplate.infrastructure.cluster.x-k8s.io/test-vpc-crs-removal-control-plane created
machinedeployment.cluster.x-k8s.io/test-vpc-crs-removal-md-0 created
ibmvpcmachinetemplate.infrastructure.cluster.x-k8s.io/test-vpc-crs-removal-md-0 created
kubeadmconfigtemplate.bootstrap.cluster.x-k8s.io/test-vpc-crs-removal-md-0 created
clusterresourceset.addons.cluster.x-k8s.io/crs-cloud-conf created
configmap/ibm-cfg created
secret/ibm-credential created
configmap/cloud-controller-manager-addon created
amanshrivastava@Amans-MacBook-Pro cluster-api-provider-ibmcloud % kubectl get clusterresourceset
NAME             APPLIED   AGE
crs-cloud-conf   True      11m
amanshrivastava@Amans-MacBook-Pro cluster-api-provider-ibmcloud % kubectl get machines
NAME                                       CLUSTER                NODE NAME                                  READY   AVAILABLE   UP-TO-DATE   PHASE     AGE     VERSION
test-vpc-crs-removal-control-plane-mzpg7   test-vpc-crs-removal   test-vpc-crs-removal-control-plane-mzpg7   False   False       True         Running   5m53s   v1.33.0
test-vpc-crs-removal-md-0-hgc8l-9lrdw      test-vpc-crs-removal   test-vpc-crs-removal-md-0-hgc8l-9lrdw      False   False       True         Running   5m46s   v1.33.0
amanshrivastava@Amans-MacBook-Pro cluster-api-provider-ibmcloud % kubectl get cluster
NAME                   CLUSTERCLASS   AVAILABLE   CP DESIRED   CP AVAILABLE   CP UP-TO-DATE   W DESIRED   W AVAILABLE   W UP-TO-DATE   PHASE         AGE   VERSION
test-vpc-crs-removal                  False       1            0              1               1           0             1              Provisioned   10m   

[aman4433]

/test pull-cluster-api-provider-ibmcloud-verify

[Karthik-K-N]

Can you explain this step, How did you do or what command did you use?

Initialized it with the changes from this branch (clusterctl init).

[aman4433]

Can you explain this step, How did you do or what command did you use?

Initialized it with the changes from this branch (clusterctl init).

Since this PR only removes the deprecated flag from documentation and CI scripts (and does not modify the Go code or manager binary), I did not build a custom image.

I used clusterctl init --infrastructure ibmcloud to initialize the standard provider. The verification focus was to ensure that omitting the EXP_CLUSTER_RESOURCE_SET=true variable (which this PR removes from ci-e2e.sh) does not break cluster creation.

I confirmed that even without exporting this flag locally, the ClusterResourceSet was successfully created and applied, validating that the explicit enablement is no longer required.

[Karthik-K-N]

Got your point, But did you check with earlier CAPIBM/CAPI release version where when you try to create without exporting the variable , will it fail