Add lock_on_merge column for labels table (redux)

What does this MR do and why?

Adds a lock_on_merge boolean column to the labels table. This is one part of the implementation of https://gitlab.com/gitlab-org/gitlab/-/issues/408676+

The original MR had to be nullified due to a problem during deployment. See Mixed deployment issue: Column added on `labels... (#421145 - closed)

Once Override attributes to include known column nam... (!128578 - merged) is merged, we can re-attempt the migration.

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