Clean up README and docstrings

[epretti]

Updates/removes some outdated information. Per #434, the documentation for SystemGenerator should reflect the fact that template generators might take more than just a string to specify a force field. Also, passing in objects other than Molecules was never supported, and this has already been removed from other docstrings, so it's removed in SystemGenerator as well.

The README also referred to some outdated information including ancient versions of OpenMM, and should now reflect that 8.5 will be required. This PR shouldn't be merged until the 8.5.1 patch release is out.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.73%. Comparing base (7b765ec) to head (ba45a36).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #435   +/-   ##
=======================================
  Coverage   84.73%   84.73%           
=======================================
  Files           5        5           
  Lines         799      799           
=======================================
  Hits          677      677           
  Misses        122      122           

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