Use CMake find_package for samples when fmt is external to Cantera

[bryanwweber]

Changes proposed in this pull request

• Bump CMake minimum version for samples to 3.5 to resolve a deprecation warning from CMake
• Find fmt with find_package() if Cantera is built with fmt as an external library

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

Closes #1742

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

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.04%. Comparing base (9210b08) to head (bcee6c7).
Report is 33 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1744   +/-   ##
=======================================
  Coverage   73.03%   73.04%           
=======================================
  Files         381      381           
  Lines       54132    54164   +32     
  Branches     9233     9240    +7     
=======================================
+ Hits        39534    39562   +28     
+ Misses      11635    11633    -2     
- Partials     2963     2969    +6     

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[ischoegl]

Thanks - lgtm!