Adds do_not_enforce_on_create option for github_repository_ruleset

[nenosrep]

Optional attribute added to rules.required_status_checks. Resolves #2656

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Before the change?

• The option to allow repositories and branches to be created without enforcing status checks were prohibited by default, and would substitute the value of this configuration from the repo to always be false. It was not available for organization rulesets in the schema, which lead to a panic.

After the change?

• Now it has the option to enable/disable enforce status checks on creation of a new branch/repo, manageable through the provider and without overwriting the configuration in the ruleset without being shown in terraform plan/apply.
• Defining required checks will no longer result in a panic, as the schema now provides a default value instead of nil.

Pull request checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] 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
• [x] No

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