docs: Update GMK resource examples now that Clusters and Topic resources are GA

[jessdejong]

Description

GMK Clusters and Topics are getting promoted to GA after the submission of GoogleCloudPlatform/magic-modules#12264. We need to update our public documentation to use our GA resources instead of the google-beta ones. Issue: https://b.corp.google.com/issues/377724373

Checklist

Readiness

• Please approve this PR, but wait submit after Promoting Managed Kafka Cluster and Topic resources to GA GoogleCloudPlatform/magic-modules#12264 is submitted.

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/managed-service-for-apache-kafka/docs/create-cluster#create

• 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

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[jessdejong]

This may need (but I might not be able to successfully trigger this).

/gcbrun

[johndee7310]

LGTM! Ship it for GA