Make merge to refs/merge-requests/:iid/merge not raise when FF-only enabled

What does this MR do?

1. Stop failing if the project merge method configuration is set to FF-only. Currently we're using the regular merge (with squash, if set). We're looking forward to support FF in https://gitlab.com/gitlab-org/gitlab-ce/issues/58226 %11.10
2. Along with the created commit_id we're also retuning the source_id and target_id (respectively source branch HEAD SHA and target branch SHA used in the commit creation). CI-team needs reliable information of the source and target IDs in order to know they're running an outdated pipeline (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9622#note_145785333).

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58393

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer