Update rake tasks to have multi-db versions

What does this MR do and why?

Update rake task mark_migration_complete to support multiple databases. Related to #352770 (closed)

If only a single database is configured, you should only see the singular rake task gitlab:db:mark_migration_complete.

If multiple databases are configured, you should see:

rake -T gitlab:db:mark_migration_complete
rake gitlab:db:mark_migration_complete[version]       # GitLab | DB | Manually insert schema migration version
rake gitlab:db:mark_migration_complete:ci[version]    # Gitlab | DB | Manually insert schema migration version on ci database
rake gitlab:db:mark_migration_complete:main[version]  # Gitlab | DB | Manually insert schema migration version on main database

How to set up and validate locally

If you have a multi-database setup, you can test the individual tasks:

1. Against main only:

   # rails gitlab:db:mark_migration_complete:main\[20220201120000\]
   Successfully marked '20220201120000' as complete on database main
   
   psql# select count(*) from schema_migrations where version = '20220201120000'; -- on main
    count
   -------
        1

2. Against ci only:

   # GITLAB_USE_MODEL_LOAD_BALANCING=true rails gitlab:db:mark_migration_complete:ci\[20220201120000\]
   Successfully marked '20220201120000' as complete on database ci
   
   psql# select count(*) from schema_migrations where version = '20220201120000'; -- on ci
    count
   -------
        1

3. Against all databases:

   # GITLAB_USE_MODEL_LOAD_BALANCING=true rails gitlab:db:mark_migration_complete\[20220201120001\]
   Successfully marked '20220201120001' as complete on database main
   Successfully marked '20220201120001' as complete on database ci
   
   psql# select count(*) from schema_migrations where version = '20220201120001'; -- on main and ci both
    count
   -------
        1

MR acceptance checklist

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