Minimum CMake downgrade for Linux (#39044)

[jashook]

Port Minimum CMake downgrade for Linux back to preview 8. This addresses #38755 and unblocks source-build in preview 8 using cmake < 3.15.

/cc @jkoritzinsky @janvorli @jeffschwMSFT @mmitche

[jashook]

Similar to the master branch I will validate that the build works and a run pri0 tests. Will update with results.

Clean builds with the following:

Os	Cmake	Clang/llvm
Centos 7	3.6.2	7
Centos 8	3.11.4	8

I have run Pri0 tests on both platforms. The following tests fail:

/runtime/artifacts/tests/coreclr/Linux.x64.Checked/Loader/binding/tracing/BinderTracingTest.ResolutionFlow/BinderTracingTest.ResolutionFlow.sh (30 seconds)
/runtime/artifacts/tests/coreclr/Linux.x64.Checked/Loader/binding/tracing/BinderTracingTest.Basic/BinderTracingTest.Basic.sh (30 seconds)
/runtime/artifacts/tests/coreclr/Linux.x64.Checked/profiler/unittest/getappdomainstaticaddress/getappdomainstaticaddress.sh (2 seconds)
/runtime/artifacts/tests/coreclr/Linux.x64.Checked/tracing/eventpipe/eventsourceerror/eventsourceerror/eventsourceerror.sh (1 seconds)

[jashook]

Manually validated the build on centos7 and centos8.

[mmitche]

@jashook Any concern with the OSX failure? I've retried it a ton of times.

[jashook]

No concern. I believe we need to port back the timeout change we made in master now that the osx machines are slower.