Run tests on .NET 8

[bricelam]

@roji I expect we'll need to react to the benchmark changes somewhere...

[ghost]

Hello @bricelam!

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[ghost]

Apologies, while this PR appears ready to be merged, I've been configured to only merge when all checks have explicitly passed. The following integrations have not reported any progress on their checks and are blocking auto-merge:

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These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

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[bricelam]

dyld : error : Symbol not found: __ZTTNSt3__114basic_ifstreamIcNS_11char_traitsIcEEEE
  Referenced from : error : /Users/runner/work/1/s/.packages/grpc.tools/2.53.0/tools/macosx_x64/protoc (which was built for Mac OS X 12.5)
  Expected in : error : /usr/lib/libc++.1.dylib
   in /Users/runner/work/1/s/.packages/grpc.tools/2.53.0/tools/macosx_x64/protoc

/cc @JamesNK

[bricelam]

CI version of macOS: 11
.NET 8 min required version: 10.15

[bricelam]

Failed with lots of new trimming warnings /cc @roji @ajcvickers

[JamesNK]

It's a known issue in Grpc.Tools that it doesn't work in macOS 11: dotnet/aspnetcore#47336

[bricelam]

Superseded by PR #31270