Define minimal K8s version in documentation, use it for e2e tests

1. Different documentation files claim different minimal K8s versions:

• deploy/helm-charts/asya-actor/README.md -> 1.24+
• deploy/helm-charts/{asya-gateway,asya-operator}/README.md -> 1.19+
• docs/quickstart/for-platform-engineers.md -> no min version
• src/asya-operator/README.md -> 1.23+
• testing/e2e/charts/asya-test-actors/README.md -> 1.19+

2. Getting warning when running e2e tests:

WARNING - Running on unsupported Kubernetes version "1.27". KEDA 2.18 is supported and tested on Kubernetes "1.31" or higher. See https://keda.sh/docs/latest/operate/cluster/ for details.

Tasks:

1. research minimal possible version of K8s that Asya should support (mostly defined by KEDA and/or its other components)
2. align documentation everywhere
3. use it for e2e tests