migrations: clear the dirty flag in each migration

Created by: chrismwendt

See https://github.com/sourcegraph/sourcegraph/issues/25089 (this PR will remain a draft unless and until we decide to move forward with this approach).

Prior to this change, it was possible for a migration to complete successfully but leave the dirty flag set to true.

After this change, each migration will set the dirty flag to false within the transaction, so that the dirty flag is cleared even if the frontend disconnects halfway through ✨

Script I used to update all the migrations: https://gist.github.com/chrismwendt/80c8f68ce12bc9c9e6638bd22971d581#file-clear-dirty-flag-py