feat: fix handling of gitlab repos

[sarahetter]

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Summary

Fixes #3771

Also not described in the issue was that we weren't displaying the correct webhook for gitlab users to add manually since we were passing through manual instead of gitlab - now it actually shows Configure the following webhook for your repository: https://api.netlify.com/hooks/gitlab as it should (and was in the code, just wasn't being exercised).

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[github-actions]

📊 Benchmark results

Comparing with de4d92d

• Dependency count: 1,073 (no change)
• Package size: 325 MB (no change)
• Number of ts-expect-error directives: 364 (no change)