NET core target to return IReadonlySet for freeze

[JR-Morgan]

Net 5 introduced the IReadOnlySet<T> interface which inherits IReadOnlyCollection<T>.
Since we now have NET8 only consumers (IFC importer), it would be nice to use this new interface

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.94%. Comparing base (87ae033) to head (2222905).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #233       +/-   ##
===========================================
+ Coverage   59.85%   70.94%   +11.09%     
===========================================
  Files         263      279       +16     
  Lines       10540    11256      +716     
  Branches     1108     1109        +1     
===========================================
+ Hits         6309     7986     +1677     
+ Misses       3970     2918     -1052     
- Partials      261      352       +91     

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[JR-Morgan]

Motivation:
IFC parser, I want to use a IReadOnlySet to make it clear its cheap to do contains, and unnecessary to do distinct, but since Freeze returns