Add latest .NET 8 runtime to global.json

Since we updated to a .NET 9 SDK, the
MS.ANC.Razor.Microbenchmarks.Generator unit tests have failed to run
locally. This is because these test use Benchmark.NET, which performs a
builder at runtime targeting .NET 8. This works fine in CI because it
uses a .NET 8 SDK.

This change just adds the most recent .NET 8 runtime version to the
list of runtimes in Razor's global.json. So, it'll get installed into
the .dotnet folder and the Microbenchmarks.Generator will run and stop
reporting a spurious error.

global.json

@@ -5,7 +5,8 @@
       "dotnet": [
         "2.1.30",
         "$(MicrosoftNETCoreBrowserDebugHostTransportPackageVersion)",
-        "3.1.30"
+        "3.1.30",
+        "8.0.7"
       ]
     },
     "vs": {