Rails5 MySQL fix rename_column as part of cleanup_concurrent_column_type_change for timestamps

What does this MR do?

There was a problem with CleanupAddTimezoneToIssuesClosedAt post migration on Rails 5 MySQL. The rename_column as part of cleanup_concurrent_column_type_change was executing ALTER TABLE `issues` CHANGE `closed_at_for_type_change` `closed_at` datetime so changing the new timestamp column back to a datetime(introduced here in rename_column_sql).

This change disables the aliasing of timestamp to datetime as part of new_column_definition. It now executes ALTER TABLE `issues` CHANGE `closed_at_for_type_change` `closed_at` timestamp NULL

This can easily be tested by running: RAILS5=1 bin/rspec spec/migrations/migrate_issues_to_ghost_user_spec.rb:20 spec/lib/gitlab/database/migration_helpers_spec.rb:1184 which will failed with

 1) Gitlab::Database::MigrationHelpers#rename_column_using_background_migration renames a column using a background migration
     Failure/Error:
       add_column(table, new_column, new_type,
                  limit: old_col.limit,
                  precision: old_col.precision,
                  scale: old_col.scale)

       #<ActiveRecord::Migration:0x00005591a6888438 @name="ActiveRecord::Migration", @version=nil, @connection=nil> received :add_column with unexpected arguments
         expected: ("issues", :closed_at_timestamp, :datetime_with_timezone, {:limit=>anything, :precision=>anything, :scale=>anything})
              got: ("issues", :closed_at_timestamp, :datetime, {:limit=>nil, :precision=>0, :scale=>nil})
       Diff:
       @@ -1,5 +1,5 @@
        ["issues",
         :closed_at_timestamp,
       - :datetime_with_timezone,
       - {:limit=>anything, :precision=>anything, :scale=>anything}]
       + :datetime,
       + {:limit=>nil, :precision=>0, :scale=>nil}]
     # ./lib/gitlab/database/migration_helpers.rb:642:in `rename_column_using_background_migration'
     # ./spec/lib/gitlab/database/migration_helpers_spec.rb:1228:in `block (3 levels) in <top (required)>'

See https://gitlab.com/jlemaes/gitlab-ce/-/jobs/80193979

Are there points in the code the reviewer needs to double check?

Is there a better way than this monkey patch? Should I add specs for this?

Why was this MR needed?

rails5 upgrade

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

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