feat: Fixup repository environment logic

[stevehipwell]

Resolves #ISSUE_NUMBER

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Before the change?

• github_repository_environment resource logic for reviewers had incorrect constraints
• github_repository_environment_deployment_policy resource missing policy ID attribute

After the change?

• github_repository_environment has correct constraints for reviewers, including validation for the sum of users and teams
• github_repository_environment_deployment_policy has new policy_id attribute
• Improved tests
• Improved repository diff logic to support gradual adoption of repository_id

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[stevehipwell]

@deiga this is ready for a re-review. I've made the most of your testing improvements. I also have thoughts around the use of tflog.SetField(); in as much as we should use tflog.SetField(d.GetId()) by default and then not use it unless there is a specific reason to do so.

[stevehipwell]

@deiga thanks for the tflog.SetField() correction. FYI there are still some instances of this pattern in the EMU group mapping resource.

[deiga]

Looking good!