Do not use 'old' OpenFF Toolkit API

[mattwthompson]

PR Summary:

A change I made to the toolkit (https://github.com/openforcefield/openff-toolkit/pull/1661) involved changing how attributes are looked up, which made this check

        uses_old_api = hasattr(Topology(), "_topology_molecules")

no longer produce the same behavior as when I wrote it. With 0.14.4, released last night, this line erroneously evaluates to True. There would be a few ways to fix this, but I'm proposing to just remove the dual support. We are no longer supporting the "old" (< 0.11) versions of the toolkit. Note, of course, that this change wouldn't affect any released version of Foyer.

I also noticed tests aren't run in parallel - it's a small and straightforward fix to get CI to use the multiple cores that GHA provides . I'm lazy.

PR Checklist

---

• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #545 (81085c5) into main (d2608b7) will increase coverage by 0.33%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #545 +/- ## ========================================== + Coverage 69.21% 69.55% +0.33% ========================================== Files 16 16 Lines 1673 1662 -11 ========================================== - Hits 1158 1156 -2 + Misses 515 506 -9 ```

[mattwthompson]

The only constraint is openff-toolkit >=0.11 but that's already in there. The quirk of 0.14.4 wasn't an API break so it should work all the way back to 0.11

[mattwthompson]

Thanks @daico007! Anything I can do to help a release? (If you have a checklist/process in place I'm happy to do as much of it as you are comfortable dumping off)

[daico007]

No problem @mattwthompson. I will put a out a patch release for foyer Monday (I may attempt to fix the current Dockerfile before that but there's a good chance it fill fail again)