Make `db:check-migrations` comment with migration output

What does this MR do and why?

Describe in detail what your merge request does and why.

As a follow-up for !102309 (merged) - Have the db:check-migrations job post a comment with the migration output, so that neither the reviewer nor the author needs to go manually check or copy-paste the migration output.

See the test output on this MR.

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