Use has_table_privilege for TRIGGER on PostgreSQL

What does this MR do?

This fixes Gitlab::Database::Grant.create_and_execute_trigger? so it uses has_table_privilege() on PostgreSQL. This was suggested in comment https://gitlab.com/gitlab-org/gitlab-ce/issues/38634#note_53356552.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/38634