Add documentation for migrating from `Required` block to new validators

[austinvalle]

Module version

v1.1.1

Use-cases

Currently, blocks that are written in SDKv2 can have a Required field applied, but there is no equivalent field or validator for SingleNestedBlock, SetNestedBlock, or ListNestedBlock. We are adding validators to solve this issue, but we should document this in our migration guide.

Proposal

• After creating the new validators, document in: https://developer.hashicorp.com/terraform/plugin/framework/migrating/attributes-blocks/blocks
• Mention the validators that can be used with each block type as well as plug to not use blocks if possible 😄

References

• Add "not null" validator for listvalidator, setvalidator, and objectvalidator terraform-plugin-framework-validators#105

[bflad]

Now that terraform-plugin-framework-validators v0.10.0 is released with the functionality for this, it may be worth including this as part of #660

[github-actions]

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