Clean up MCPL references

[tjlaboss]

Description

• Revise MCPL instructions in installation guide to reflect that it is an optional runtime dependency (see Make MCPL a Runtime Optional Dependency #3429) and remove leftover reference to its CMake option.
• Remove "MCPL support" from version output.

Fixes #3926

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)