gitlab-housekeeper skip updates if edited by another person

What does this MR do and why?

The gitlab-housekeeper gem is a tool for automatically creating merge requests for repetitive things like finalizing batched background migrations. It was introduced in !139492 (merged) .

For now this is just a tool developers can run locally to automate the MR creation. Longer term it will be run in CI. It is not actually part of the running GitLab application.

Sometimes the generated merge request needs to be amended by a developer (eg. pipeline fails due to MR title too long). In those cases we don't want to overwrite their changes.

This merge request extends the gem to look through the system notes on the MR and detect if the title, description or code has been edited by anyone else. If that's the case it does not update that particular detail of the merge request.

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