Add prevent_self_review parameter to the github_repository_environment resource

[Nmishin]

Resolves https://github.com/integrations/terraform-provider-github/issues/1967

Before the change?

• We can't manage "Prevent self-review" option

After the change?

• We can manage "Prevent self-review" option

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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[kfcampbell]

2011 will unblock this PR.

[andreas-aman]

I saw #2011 has been superseeded by #2081 that has been merged. Does this mean this PR can be unblocked?

[Nmishin]

@kfcampbell MR updated, could you please review