Switching to the modern runtime would be good, because the old runtime is missing features, which has meant writing workarounds for Windows in our code: i.e #29014/#29357.

Mingw-w64 12.0.0 has been released, which now defaults to the UCRT runtime, over MSVCRT. See https://github.com/mingw-w64/mingw-w64/blob/master/mingw-w64-doc/howto-build/ucrt-vs-msvcrt.txt for more details. This makes using it somewhat easier, because we can just use 12.0.0 verbatim, rather than having to configure an older mingw-w64 for the newer runtime.

• Upstream mingw-w64 12.0.0 into Guix: See https://git.savannah.gnu.org/cgit/guix.git/commit/?id=17188be0f723e00377b21b767f5447d7938a116e.
• Update Guix time-machine here: guix: bump time-machine to efc26826400762207cde9f23802cfe75a737963c #30452.
• Add CI to cover new runtime.
• Drop MSVCRT workarounds.
• Any other documentation / Windows release build configuration updates.
• A symbol-check that the new runtime is indeed being used.