Document db:migrate:status rake task

What does this MR do?

Maintenence rake task list lacks information on gitlab-rake db:migrate:status command, used to check the status of migrations.

This command can be very helpful, but it is not documented.

MR documents use of the rake task and expected output.

Fix #63908 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides