cmake: set CMAKE_CXX_STANDARD 17 to match code assumptions

[erkki]

Usage of std::in_place_t implies C++17

Build on macOS 15.3.2 fails otherwise with:

/opt/homebrew/Cellar/abseil/20250127.1/include/absl/utility/utility.h:85:12: fatal error: no member named 'in_place_t' in namespace 'std'
   85 | using std::in_place_t;
      |       ~~~~~^
1 error generated.

[dagar]

This should be fine, but let's take a closer look at a Nuttx build for any differences and get it through CI cleanly.