Faster CI: Switch to ninja as generator

We are currently using the following generators:

• MacOSX: ninja
• Linux: "Unix Makefiles"
• Windows: VS 2022

ninja is known to be really fast and I also use ninja internally on all platforms.

We could move to using ninja also on Linux/Windows. If we are concerned with breaking multi-config generators we can also use "Ninja Multi-Config" somewhere (Windows?).