WIP: CI(linux): switch LTS versions #1782

Job	Run time
linux-newLTS-vanilla--dotnet8	3m 39s
linux-newLTS-github--dotnet-and-xbuild	3m 58s
linux-newLTS-github--dotnet-and-msbuild	4m 23s
linux-newLTS-vanilla--newmono	5m 19s
linux-newLTS-vanilla--stockmono	4m 7s
linux-oldLTS-github--dotnet-and-msbuild	9m 20s
linux-oldLTS-github--dotnet-and-xbuild	8m 59s
linux-oldLTS-vanilla--newmono	5m 15s
linux-oldLTS-vanilla--stockmono	4m 19s
macOS--dotnet6-and-mono	9m 9s
macOS--mono	5m 3s
windows--dotnet6	1m 0s
windows--legacyFramework	7m 38s
sanity-check	0s
publish	0s
	1h 12m 9s

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