feat: Clarify maintenance exclusion usage in no channel to release channel samples for GKE

[cslink]

Description

Fixes # b/396134995

Note: If you are not associated with Google, open an issue for discussion before submitting a pull request.

Checklist

Readiness

• Yes, merge this PR after it is approved
• No, don't merge this PR after it is approved

Style

• My sample follows the rules described for Terraform in the Effective Samples style guide
• My sample follows the other requirements and best practices in the Contributing
  guide

Testing

• I have performed tests described in the Contributing guide:

  • Tests pass: terraform apply
  • Lint pass: terraform fmt check

Intended location

• Yes, this sample will be (or already is) included on cloud.google.com
  Location(s): https://cloud.google.com/kubernetes-engine/docs/how-to/release-channels

• No, this sample won't be included on cloud.google.com
  Reason:

API enablement

• If the sample needs an API enabled to pass testing, I have added the service to the Test setup file

Review

• If this sample adds a new directory, I have added codeowners to the CODEOWNERS file

[cslink]

Tagging @apeabody. Do you know how to fix the diff? I would assume that PR #820 should already be merged, unsure why that isn't being taken into account.

[apeabody]

/gcbrun