Resolve "Squash and merge in GitLab Core (CE)"

What does this MR do?

Add Squash and merge feature from EE to CE.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The most popular feature proposal.

Screenshots (if relevant)

To be added.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #34591 (closed)