Add 14.9 to upgrade path

What does this MR do?

This MR cherry picks the commits from !83918 (closed) to add 14.9 to the upgrade path.

The following issue was discussed in the Database team meeting yesterday, and the conclusion was that 14.9 does need to be on the upgrade path. There was also a lot of discussion about how to avoid this in the future (see #360556 (closed)).

Caused by: Expected batched background migration for the given configuration to be marked as 'finished', 
but it is 'active': {:job_class_name=>"ProjectNamespaces::BackfillProjectNamespaces", 
:table_name=>:projects, :column_name=>:id, :job_arguments=>[nil, "up"]} Finalize it manualy by 
running sudo gitlab-rake gitlab:background_migrations:finalize[ProjectNamespaces::BackfillProjectNamespaces,projects,id,'[null\,"up"]'] 
For more information, check the documentation 

More context for this update can also be be found in my comment on the related issue: omnibus-gitlab#6795 (comment 926026956)

Related issues

• omnibus-gitlab#6795 (closed)

From the original MR:

• #355317 (closed)

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