Include dotnet 3.1, 6.0 and 8.0 in devcontainer #22

Job	Run time
tests (ubuntu-latest, 3.1.x)	33s
tests (ubuntu-latest, 6.0.x)	33s
tests (ubuntu-latest, 8.0.x)	37s
tests (macOS-latest, 6.0.x)	29s
tests (macOS-latest, 8.0.x)	35s
tests (windows-latest, 3.1.x)	3m 44s
tests (windows-latest, 6.0.x)	2m 53s
tests (windows-latest, 8.0.x)	2m 3s
integrationTests (ubuntu-latest, 3.1.x)	31s
integrationTests (ubuntu-latest, 8.0.x)	37s
integrationTests (macOS-latest, 8.0.x)	24s
integrationTests (windows-latest, 3.1.x)	2m 10s
integrationTests (windows-latest, 8.0.x)	2m 10s
publish	0s
	17m 19s

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