Remove validate_merge_sha feature flag

What does this MR do?

This removes the validate_merge_sha feature flag that was added in !19579 (merged) which was included in 12.5

Starting 12.5 we started writing the merge_sha inside the merge params for validating.

To avoid merges failing during a deploy, the feature flag wasn't defaulted to "on". But one release later we can remove it since the SHAs will would have been written in the params regardless.

Closes #27824 (closed)

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• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content