[docs] update API version for Kubernetes exec configuration to v1

[schnell3526]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

No changes to security controls. This PR updates the API version reference in documentation and examples to align with Kubernetes stable API version.

Description

This PR updates the Kubernetes exec-based credential plugin API version from client.authentication.k8s.io/v1beta1 to client.authentication.k8s.io/v1 across all documentation, examples, and test infrastructure.

The v1beta1 API version has been deprecated in favor of the stable v1 API version. This change ensures that:

• Documentation reflects current best practices
• Examples use the stable, non-deprecated API version
• Test infrastructure is aligned with modern Kubernetes clusters

Files Updated:

• Documentation files (guides, resource docs, index)
• Example Terraform configurations
• Test infrastructure for EKS and GKE

This is a documentation and example update only - no provider code logic has been changed.

Acceptance tests

• Have you added an acceptance test for the functionality being added?
  • N/A - This is a documentation/example update only
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

N/A - No provider code changes

Release Note

Release note for CHANGELOG:

NOTES:

* Documentation and examples have been updated to use `client.authentication.k8s.io/v1` instead of the deprecated `v1beta1` API version for exec-based credential plugins

References

- https://kubernetes.io/docs/reference/access-authn-authz/authentication/#client-go-credential-plugins
- The v1beta1 API version is deprecated and v1 is now the stable version for ExecCredential

Community Note

- Please vote on this issue by adding a 👍 https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/ to the original issue to help the community and maintainers prioritize this request
- If you are interested in working on this issue or have submitted a pull request, please leave a comment

[hashicorp-cla-app]

All committers have signed the CLA.