Replace M_PI by std::numbers::pi

[bernhardmgruber]

The exercises memcheck and polymorphism use the non-standard M_PI constant, which unfortunately for MSVC requires extra setup. If we would use std::numbers::pi instead from C++20, this special handling would not be necessary. However, it would move the exercises from C++17 to C++20. Would this be fine?

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.