Fix updating merge_when_build_succeeds via merge API endpoint

What does this MR do?

When updating a merge request via the /merge endpoint we check the mergeable and mergeable_state status, these will return false if the application option only_allow_merge_if_pipeline_succeeds is enabled. We should skip CI checks if the request uses the merge_when_pipeline_succeeds param

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

• specs

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/22740