Resolve "Allow the configuration of a project's merge method via the API"

What does this MR do?

Adds another optional parameter to the projects API endpoint: merge_method that allows to set the merge method of a specific project.

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

I'm not quite sure whether merge, rebase_merge and ff are appropriate identifiers for the respective merge methods, but I couldn't think of anything better and didn't want to change more than necessary.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary

• Documentation created/updated

• API support added

• Tests added for this feature/bug

• Review

  • Has been reviewed by Backend

• Conform by the merge request performance guides

• Conform by the style guides

What are the relevant issue numbers?

Closes #39880 (closed)