Fix FieldOfMatrixGroup calls, analogous to the GAP library

[ThomasBreuer]

The Forms package provides methods for SymplecticGroup, GeneralOrthogonalGroup, etc. where form objects are involved. The proposed changes adjust these methods to the fixes proposed in gap-system/gap/pull/6203.

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.97%. Comparing base (c54faa1) to head (ebbbaf1).

Files with missing lines	Patch %	Lines
lib/classic.gi	95.12%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   80.88%   80.97%   +0.08%     
==========================================
  Files           6        6              
  Lines        3663     3689      +26     
==========================================
+ Hits         2963     2987      +24     
- Misses        700      702       +2     

Files with missing lines	Coverage Δ
lib/classic.gi	99.04% <95.12%> (-0.30%)	⬇️

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