Pin openff forcefields

[IAlibay]

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 92.57%. Comparing base (733908d) to head (872e276).
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #932      +/-   ##
==========================================
- Coverage   94.32%   92.57%   -1.75%     
==========================================
  Files         134      134              
  Lines        9917     9917              
==========================================
- Hits         9354     9181     -173     
- Misses        563      736     +173     

Flag	Coverage Δ
fast-tests	92.57% <ø> (?)
slow-tests	?

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