Update SDK to 8.0.100-alpha.1.22601.6

The 8.0.100-alpha.1.22512.5 SDK was built in a window where System
references were versioned `8.0.0.0`, but in a way that broke some
things. That was backed out in dotnet/runtime#77899, which means that
assemblies built from this repo can't run on the current .NET 8 runtime,
which cannot satisfy a dependency on, for example, `System.Runtime,
Version=8.0.0.0`.

Move forward to a newer .NET 8 that has those assemblies versioned at
`7.0.0.0` again, which should work even when dotnet/runtime#78354 bumps
the version again, because the loader will accept higher versions.

Author: rainersigwald

global.json

@@ -1,6 +1,6 @@
 {
   "tools": {
-    "dotnet": "8.0.100-alpha.1.22512.5",
+    "dotnet": "8.0.100-alpha.1.22601.6",
     "runtimes": {
       "dotnet": [
         "2.1.7",