Make allows_public_repositories attribute on github_actions_runner_group modifiable

[dbaur]

Resolves #1697

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Behavior

Before the change?

• Currently the allows_public_repositories attribute on the github_actions_runner_group resource is marked as computed an thus can not be set.
• The attribute is however settable via the API (see https://docs.github.com/en/enterprise-cloud@latest/rest/actions/self-hosted-runner-groups?apiVersion=2022-11-28#create-a-self-hosted-runner-group-for-an-organization)

After the change?

• The allows_public_repositories attribute is now marked as Optional with the value defaulting to false (the same default value as described by the GitHub API)

Other information

• The Acceptance tests for the github_actions_runner_group were not working. The selected_workflows test case references a) a file that does not exist and b) uses an invalid ref. I have updated the acceptance tests ensuring that the workflow file exists in the given repo and that the correct ref to the file is used.
• One could think about updating the docs, as the ref in the current example is misleading

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[kfcampbell]

One could think about updating the docs, as the ref in the current example is misleading

Do you mind explaining a little more what you mean by this?

This PR looks great, thank you for contributing and improving the tests! The acceptance tests are passing for me locally.