Upgrade Phlex to build with C++23

[knoepfel]

This PR contains the following:

• Changes the base container image from gcc:15.2.0 (Debian-based) to ubuntu:24.04 as the latter matches the OS supported by GitHub Actions runners
• Builds GCC 15.2.0 using Spack and loads it as necessary when building phlex
• Changes the Spack environment specification to use cxxstd=23 where appropriate
• Adjust the CMake preset to use C++23

[greenc-FNAL]

Note: your container uses LLVM/Clang 21.1.8 rather than the previous 21.1.4. Would need to look closer to see if that causes an issue somehow, though.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

❌ Your project status has failed because the head coverage (74.04%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   74.28%   74.04%   -0.24%     
==========================================
  Files         124      124              
  Lines        2955     2955              
  Branches      513      513              
==========================================
- Hits         2195     2188       -7     
- Misses        540      545       +5     
- Partials      220      222       +2     

Flag	Coverage Δ
unittests	74.04% <ø> (-0.24%)	⬇️

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see 3 files with indirect coverage changes

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[beojan]

I see no issues

I believe I addressed the requested changes.

Added code comments as requested