Update docs to indicate when custom headers was added to project hooks

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This change updates the documentation for a GitLab project webhook feature to clarify when it became available. Specifically, it adds a note that the "custom headers" functionality for project webhooks is only available starting from GitLab version 17.1 and later. The update appears in both the user-facing documentation and the internal code description to ensure users know the minimum GitLab version required to use this feature.

Resolves #6601 (closed)