Add migration to remove pending delete projects with non-existing namespace

What does this MR do?

There might be some projects where the namespace was removed, but the project wasn't. For these the projects still have a namespace_id set. So this adds a post-deploy migration remove all projects that are pending delete, and have a namespace_id that is non-existing.

This is a follow-up of !11044 (merged).

Database Checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted.
  => NOOP
• Added the output of the migration(s) to the MR body => On production 129 rows would be affected, see https://gitlab.com/gitlab-com/infrastructure/issues/2548#note_38245210
• Added the execution time of the migration(s) to the MR body => On staging it took 0.24ms, see https://gitlab.com/gitlab-com/infrastructure/issues/2548#note_38222879
• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)
• Made sure the migration won't interfere with a running GitLab cluster, for example by disabling transactions for long running migrations

When adding or modifying queries to improve performance:

• Included the raw SQL queries of the relevant queries:
  => Query generated by CleanupNonexistingNamespacePendingDeleteProjects#find_batch

  SELECT "projects"."id"
  FROM "projects"
  WHERE "projects"."pending_delete" = 't'
    AND (NOT (EXISTS
                (SELECT 1
                 FROM "namespaces"
                 WHERE "namespaces"."id" = "projects"."namespace_id")))

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#36541.