Good error message when using unsupported C++ version

[bretbrownjr]

Motivation

Beman contributors, users, and maintainers will sometimes use a C++ version that is unsupported by a given project. There should be clear feedback when this happens, especially when it happens accidentally.

Completion Criteria

This issue will be complete when:

• There is clear error message is printed when using the exemplar repo with an unsupported C++ version
• The Beman Standard is updated to either recommend or require that all Beman libraries do likewise

Guidance

A user-unfriendly error message might be a compiler returning an error like: error: expected '(' before 'constexpr'.

Instead, we would like something more literally describing to the user that the C++ version needs to use C++23 or newer.

Mentor

@bretbrownjr is happy to discuss, review, and answer any questions.

[wusatosi]

Related: bemanproject/exemplar#40

There's no current consensus on the approach.

[wusatosi]

Honestly I would advocate for a requirement to add a section in the README/ docs that explains what is the minimum C++ requirement in current project and what would it take to lower it (basically explaining why it is what it is).

We can then add a detection CMake script in exemplar that basically denies configuration and spits out "Minimum version: xx. Read the README for why".