New repository settings to disable or restrict pull requests [GA]

### Value Prop

Repository maintainers get two new, first-party controls to manage contributions at the repo level: disable pull requests entirely or restrict PR creation to collaborators only. This lets teams share code publicly (or mirror it) without taking on the overhead and risk of unsolicited or low-quality PR submissions, and without needing custom bots or manual triage.

### Expected Outcome

Maintainers can reduce PR noise and moderation workload, while still keeping repositories accessible for viewing and forking with clear UX signals about contribution limits. Teams that want controlled contributions can allow PRs from trusted collaborators only, leading to higher-signal contribution flows, fewer unwanted PRs, and smoother maintenance—especially during pauses, release gaps, or in mirrored/readonly repos.