Cleanups to README.md and CONTRIBUTING.md

[ednolan]

• Update CMake version in README
• Cookiecutter: Fix a missed minimum C++ version variable
• Remove language about C++20 range requirements in tests
  • This should be obvious to users if and when they encounter the error, and this language kept being copied into the READMEs of other libraries when it wasn't relevant.
• Remove CMAKE_PREFIX_PATH parameter from CONTRIBUTING.md documentation
  • This hasn't been necessary since 2a8725d but the update to the README was missed.
• Refactor "Integrate beman.exemplar into your project" README section
  • This section is now split up into four sections: Build, Installation, CMake Configuration, and Using beman.exemplar. Some of the verbiage has been rewritten.
• Remove GitHub Codespace language from CONTRIBUTING.md
  • I am sunsetting the Beman project's use of GitHub codespaces due to maintenance burden, and a poor developer experience caused by excessive load times for launching the codespace instances.
• Remove language around installing build requirements
  • Information about how to install CMake, LLVM, and MSVC is a quick Google search away. This document is more valuable if we stick to the information that's specifically relevant to building Beman libraries.
• Refactor language in CONTRIBUTING.md
  • There are no major changes to the substance of what the document communicates in this commit, but it rephrases some of the language to make it clearer.

[coveralls]

coverage: 100.0%. remained the same
when pulling ba11199 on enolan_readmecleanup1
into e0431ac on main.