Drop MSVC 2019 support, support 2026 instead

This currently doesn't work for the standard MSBuild-based build, as CMake 4.2 is required for the VS 2026 generator, and we can't support it while we're still using a Recast commit that doesn't support it. resolved by !5136 (merged) It seems to work fine for Ninja, though.

Despite reading a bunch of things that suggested the contrary, I seem to have an MSVC installation that self-identifies as 14.5x.x in my VS 2026 install, so maybe MSVC 2026/v145 is real after all.

We can't build dedicated deps for 2026 until https://github.com/actions/runner-images/issues/13291 is resolved.