fix: Correct forking and vulnerability alert logic

[stevehipwell]

Resolves #2988
Resolves #3111

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Before the change?

• Setting allow_forking = false on github_repository resulted in incorrect behaviour
• The vulnerability_alerts attribute of github_repository couldn't be left unset to support default behaviour
• Missing tests for github_repository allow forking and vulnerability alerts
• Tests from github_organization_role_users data source were broken

After the change?

• Setting allow_forking = false on github_repository works as expected
• The vulnerability_alerts attribute of github_repository works correctly when unset
• Tests for github_repository allow forking and vulnerability alerts added
• Tests from github_organization_role_users data source fixed

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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[nickfloyd]

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