ReadMe.md: revamp to remove .NETFramework/Mono #1527

Job	Run time
linux-newLTS-vanilla--dotnet6	7m 44s
linux-newLTS-github--dotnet-and-msbuild	8m 18s
linux-newLTS-vanilla--newmono	6m 38s
linux-newLTS-github--dotnet-and-xbuild	7m 32s
linux-newLTS-vanilla--stockmono	5m 10s
linux-oldLTS-github--dotnet-and-msbuild	7m 39s
linux-oldLTS-github--dotnet-and-xbuild	7m 51s
linux-oldLTS-vanilla--newmono	7m 59s
macOS--dotnet6-and-mono	11m 44s
linux-oldLTS-vanilla--stockmono	6m 32s
macOS--mono	6m 7s
windows--dotnet6	9m 25s
windows--legacyFramework	15m 32s
sanity-check	59s
publish	56s
	1h 50m 6s

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