Merge branch '372316-parallel-bbm-remove-ff' into 'master'

Remove batched_migrations_parallel_execution flag

See merge request gitlab-org/gitlab!120808



Merged-by: Krasimir Angelov <kangelov@gitlab.com>
Approved-by: Marius Bobin <mbobin@gitlab.com>
Approved-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Reviewed-by: Marius Bobin <mbobin@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Prabakaran Murugesan <pmurugesan@gitlab.com>