Resolve build infrastructure errors (with mpi4py, gams, networkx)

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves numerous several errors relating to the build/test infrastructure that all arose during the migration to supporting Python 3.12:

• openmpi installation on conda started triggering installation errors. In the end, this switches the tests to not specifying openmpi (and instead defaulting to using mpich). In the course of working on this, it also identified bugs in the logic for skipping MPI tests.
• networkx had a broken build pushed to pypi that enabled installation of 3.2 on Python 3.8 (3.2 requires Python >= 3.9). This refines our optional requirements to explicitly exclude networkx 3.2 on Python < 3.9. This highlighted a bug in how we were doing version comparison in setup.py
• This tracks a move in a GAMS import

Changes proposed in this PR:

• switch MPI tests to not use openmpi
• fix logic for skipping MPI tests
• restrict the networkx version for python<3.9
• update GAMS import of GamsExceptionExecution

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4267cbd) 87.92% compared to head (0593891) 87.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3018      +/-   ##
==========================================
- Coverage   87.92%   87.84%   -0.09%     
==========================================
  Files         769      769              
  Lines       89538    89541       +3     
==========================================
- Hits        78727    78657      -70     
- Misses      10811    10884      +73     

Flag	Coverage Δ
linux	85.17% <0.00%> (-0.01%)	⬇️
osx	74.97% <0.00%> (-0.01%)	⬇️
other	85.35% <0.00%> (-0.01%)	⬇️
win	82.40% <0.00%> (-0.01%)	⬇️

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Files	Coverage Δ
pyomo/solvers/plugins/solvers/GAMS.py	60.66% <50.00%> (-6.91%)	⬇️

... and 6 files with indirect coverage changes

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