Resolve "Remove two_step_rebase feature flag"

What does this MR do?

Removes the feature flag two_step_rebase as outlined in #29077 (closed)

Makes gitlab-foss!27446 (merged) official!

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related MR (to be merged after this): !23880 (merged)

Closes #29077 (closed)