Fix column differences in `db/schema.rb`

What does this MR do?

project_custom_attributes and group_custom_attributes used datetime instead of datetime_with_timezone column type in database schema. But actually migration defines t.timestamps_with_timezone.

Reproducible by:

• re-create DB and run migrations on master branch
• running rake db:schema:dump generates some differences in schema.rb, except shuffled columns there are also differences addressed by this MR

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides