Remove the validate_merge_sha feature flag

We could not have this feature flag enabled by default, to avoid failing merges during a deploy, so this would need to wait until %12.6. At that moment we also need to add the changelog.

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The following discussion from !19579 (merged) should be addressed:

• @reprazent started a discussion:

  We need to keep this default_enabled to false for now on. Since otherwise the merge buttons clicked during a deploy would fail. We can enable it after a deploy and all will be well, since all buttons clicked afterwards will store the sha in the merge_params.

  Leaving this discussion open to create a follow up issue to remove the feature flag.