Point docfx metadata extraction at net8.0 instead of net10.0

docfx 2.74.1 is itself a net8.0 tool; its metadata-extraction step
does its own internal design-time build that fails with NETSDK1045
when told to target net10.0, regardless of which SDKs are installed
on the runner. net8.0 is one of RxSharp's own multi-targeted TFMs and
docfx's native runtime, so this sidesteps the mismatch entirely
instead of just installing more SDKs (which didn't help).

Author: kblok

docfx_project/docfx.json

@@ -12,7 +12,7 @@
       "dest": "api",
       "disableGitFeatures": false,
       "properties": {
-        "TargetFramework": "net10.0"
+        "TargetFramework": "net8.0"
       },
       "allowCompilationErrors": true
     }