Fix API issues sorting

What does this MR do?

Fix the sorting of issues in the API.

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

Instead of removing the '_at' suffix manually, we could add those versions to the Sortable concern instead.

Why was this MR needed?

There were a couple of bugs:

• The global and project-specific issues endpoints wouldn't sort at all.
• Group sorting would work, but only if you applied two undocumented workarounds:
  • Always pass both order_by and sort (both are optional, so only one should be needed to change ordering).
  • Instead of passing created_at or updated_at, you needed to pass created or updated.

This makes the API implementation match the docs.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/983.