[AMDGPU] Remove Code Object V3 (#67118)

V3 has been deprecated for a while as well, so it can safely be removed
like V2 was removed.

- [Clang] Set minimum code object version to 4
- [lld] Fix tests using code object v3
- Remove code object V3 from the AMDGPU backend, and delete or port v3
tests to v4.
- Update docs to make it clear V3 can no longer be emitted.