Add new step enable mirror via UI

What does this MR do?

Fixing stale test pipeline_for_project_mirror_github_spec.rb. We now need to enable "Trigger pipeline for mirror updates" in UI first before sending trigger request to the API. Please see conversation #335045 (comment 618356404) for more in detail analysis.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #335045 (closed)