Resolve compilation issues with C++20 mode and fmt 11.0

[speth]

Changes proposed in this pull request

• Fix usage of fmt when compiling in C++20 mode
• Add support for the recently-released fmt 11.0
• Add CI tests specifying use of C++20 mode and using fmt 11.0

If applicable, fill in the issue number this pull request is fixing

• Closes #1711
• Closes #1735

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.03%. Comparing base (6931eb3) to head (03177f5). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/base/SolutionArray.cpp	96.15%	1 Missing :warning:
src/thermo/PureFluidPhase.cpp	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1739 +/- ## ======================================= Coverage 73.03% 73.03% ======================================= Files 381 381 Lines 54128 54132 +4 Branches 9233 9233 ======================================= + Hits 39530 39534 +4 Misses 11635 11635 Partials 2963 2963 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.