Fix Codespaces prebuild after CMake upgrade and set policy in Mono #88410
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The prebuild on Codespaces broke after the minimum CMake version was bumped in #86530 because the container was still using Ubuntu 20.04 (Focal) which only has CMake 3.16.1.
Upgrade to Ubuntu 22.04 (Jammy) so we have a new enough CMake.
Also fix a missing policy in the Mono CMakeLists.txt that was accidentally removed.