Add step for testing on the released server

[greschd]

Add a step to the tests on Python 3.12 for testing with the 2024R2 server version. [1]

Expected test failures (new features) are marked with the new xfail_before fixture. The
only current expected failure is the modeling ply geometry export test (added in 25.1).

Also fixes an issue with the clone method when unlink=True is specified: Any
fields unknown to the current client were retained. This caused an error on storing,
since those may be unknown linked objects. In the current case, the newly added
links to the Fabric caused this.
Calling DiscardUnknownFields() on the protobuf message fixes this.

[1] Once there are multiple released server versions to test, it probably makes sense to
put these tests into separate jobs.

[greschd]

Failing due to #574, I will re-create the tagged container from the customer portal artifacts.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.91%. Comparing base (b4bba30) to head (9e18580).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #573   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files          82       82           
  Lines        4292     4293    +1     
=======================================
+ Hits         3988     3989    +1     
  Misses        304      304           

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