Clean up schema for table protected_tag_create_access_levels

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Per https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: UNIQUE (protected_tag_id)

Unused columns (?):

• user_id is always NULL
• group_id is always NULL

These columns may be NULL because nobody on GitLab.com uses them. If they are not used by the application at all we can remove them.