Unpin GAMS release; rebuild GHA package cache

[jsiirola]

Fixes # .

Summary/Motivation:

The Demo license for the pinned GAMS version in GHA has expired, leading to test failures on main. This releases the pin on the GAMS version and rebuilds the GHA package cache.

Changes proposed in this PR:

• Unpin GAMS release version
• Rebuild package caches

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.11%. Comparing base (360a87e) to head (2655115).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3950      +/-   ##
==========================================
+ Coverage   86.98%   90.11%   +3.13%     
==========================================
  Files         905      905              
  Lines      107502   107502              
==========================================
+ Hits        93512    96879    +3367     
+ Misses      13990    10623    -3367     

Flag	Coverage Δ
builders	29.13% <ø> (+<0.01%)	⬆️
default	86.46% <ø> (?)
expensive	35.53% <ø> (?)
linux	87.61% <ø> (+1.03%)	⬆️
linux_other	87.61% <ø> (?)
oldsolvers	28.07% <ø> (+<0.01%)	⬆️
osx	82.99% <ø> (?)
win	85.52% <ø> (?)

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