Update list implementation in google_dns_keys data source

[modular-magician]

Closes #14298

google_dns_keys was recently migrated to the plugin framework. Currently the Google provider is muxed and uses protocol 5.

Computed blocks have issues when protocol 5 is in use, which result in the problem seen in #14298

In this PR I

1. change the DNS keys data source to follow the recommended approach for avoiding this issue
2. update acceptance tests to access these attributes, showing that these code changes fix the issue

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Useful links:

• Investigate and Determine Computed-Only Block Behavior terraform-plugin-framework#214 (comment)
• Official documentation's guidance for this issue when using Computed blocks in a muxed provider

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If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

dns: fixed bug in `google_dns_keys` data source where list attributes could not be used at plan-time

Derived from GoogleCloudPlatform/magic-modules#7748

[github-actions]

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