In SERVER-70321 we fixed a bug that was causing the CollMod coordinator to re-execute a phase after unblocking migration on the collection.

The clean way to solve that bug would have been to move the resumeMigration call to a dedicated phase. Unfortunately, since we needed to backport the fix we couldn't add a new phase otherwise we would have introduce backward incompatibility. Thus in the end we implemented a more involved solution.

The adopted solution relies on checking for the presence of the allowMigrations flag on the config server to understand if we already called resumeMigration. Unfortunately, this does not work for tracked unsharded collection (introduced in 8.0) because for this class of collection stopMigration() does not set the allowMigration flag.