Add comment when DB testing pipeline is started

What does this MR do?

Create/update a comment when the DB testing pipeline is kicked off. The comment is then updated with migration info once the testing pipeline completes (https://gitlab.com/gitlab-org/database-team/gitlab-com-database-testing/-/blob/master/notifier/notifier.rb#L16)

Test comment: !60842 (comment 575292298)

Related to gitlab-org/database-team/team-tasks#170 (closed).

Screenshots (strongly suggested)

This comment is created when the pipeline is triggered:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not a user-facing change.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done