Changes proposed in this pull request

• Update the minimum CMake version in generated CMakeLists.txt files. A minimum version of at least 3.5 is required to work with the current CMake version (3.31). I went higher than this for the sake of not having to update this for a good long while. 3.18 is the CMake version in Debian Bullseye, currently the "oldstable" release, so this shouldn't be much of a hurdle.
• Fix the use of a deprecated version of MassFlowController.setTimeFunction in combustor.cpp.

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

The first issue here is one of the problems currently preventing successful Conda package builds (see conda-forge/cantera-feedstock#63).

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