Reconcile columns that were changed from datetime_with_timezone to datetime

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https://gitlab.com/gitlab-org/gitlab-ee/commit/01b0617cb47a2f80b0f913927182f7ddb07743df reverted a number of columns from datetime_with_timezone to datetime. This has caused problems such as gitlab-org/gitlab-ee#4803.

Do we need to:

1. Revert all columns back to datetime_with_timezone (at least to be consistent with CE)?
2. Deal with migrating any columns that aren't right?

/cc: @yorickpeterse