Switch to Ninja generator for windows cmake builds.

These will still use MSVC as the compiler, but will no longer generate Visual Studio projects for the builds.  Visual Studio is particularly bad at parallelizing builds, and is hostile to ccache.

This change also tweaks the ccache setup to prevent unbounded the growth observed in our github caches.  Windows builds have had debug symbols stripped to reduce ccache size by a factor of 2x, and ccache maximum have been tweaked so that we persist fewer older builds.

Before this change, each CMake build took 12 minutes on every run (plus some constant overhead from staleness/gcloud), even with caching or on large multi-core runners.  No amount of caching or parallelization made any noticeable difference above noise.  With this change, we see the following improvements:
- 12 minutes to build from scratch on normal runners (unchanged)
- 4 minutes on 32-core runners from scratch
- 1 minute with optimal caches available on normal runners.
- 30 seconds on 32-core runners with optimal caches

PiperOrigin-RevId: 508799909