Update version.json for 8.0.0-rc pre-release

Couldn't use 8.0.230823-rc release due to assembly metadata, so need to fix that elsewhere for release, see failed build here: https://dev.azure.com/dotnet/CommunityToolkit/_build/results?buildId=97528

version.json

@@ -1,5 +1,5 @@
 {
-  "version": "7.1.2-build.{height}",
+  "version": "8.0.0-rc",
   "publicReleaseRefSpec": [
     "^refs/heads/main$", // we release out of main
     "^refs/heads/dev$", // we release out of dev