Fix remaining -Wcast-qual warnings and enable the warning in CI

[seanm]

I've fixed as many -Wcast-qual warnings as I can, but there are about 9 left (see also https://github.com/OSGeo/PROJ/pull/3496). They need someone that knows the codebase well (not me).

IMNSHO, C++ codebases these days should be free of such warnings. Even suppressing them with const_cast is better than nothing.

Once fixed, the warning should be enabled in CI to keep new such warnings from being introduced.