revert setup dotnet version bump #384

Job	Run time
linux-arm (arm64)	34m 30s
linux-compiler-compat (clang-3)	3m 23s
clang-sanitizers (,thread)	3m 48s
linux_mono	3m 19s
linux-compiler-compat (clang-6)	3m 19s
clang-sanitizers (,address,undefined)	3m 27s
linux-compat (al2-x64)	1m 24s
linux-compiler-compat (clang-8)	3m 28s
osx	3m 54s
windows-vc14 (x86)	5m 22s
windows-vc16 (x64)	6m 36s
windows-vc14 (x64)	6m 26s
linux-compiler-compat (clang-9)	3m 22s
check-submodules	37s
linux-compiler-compat (clang-10)	3m 23s
osx-arm64-cross-compile	2m 29s
linux-compiler-compat (clang-11)	3m 23s
linux-compiler-compat (gcc-4.8)	3m 19s
linux-compiler-compat (gcc-5)	3m 19s
linux-compiler-compat (gcc-6)	3m 21s
linux-compiler-compat (gcc-7)	3m 25s
linux-compiler-compat (gcc-8)	3m 16s
	1h 48m 50s

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