Fix OpenMMForceFields gaff tests version checks

[IAlibay]

Fixes #890

For now I'm leaving the ommforcefields 0.13 check, this is so that we know the tests don't fail with older versions of OpenMM. Once the next version of OpenMMForceFields is out where gaff is supported with older OpenMM versions then we should remove these checks.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.17%. Comparing base (ecc8a50) to head (72431cc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #893      +/-   ##
==========================================
- Coverage   93.98%   92.17%   -1.81%     
==========================================
  Files         134      134              
  Lines        9790     9790              
==========================================
- Hits         9201     9024     -177     
- Misses        589      766     +177     

Flag	Coverage Δ
fast-tests	92.17% <ø> (?)
slow-tests	?

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[IAlibay]

Will deal with mypy failure elsewhere