Update version.json to 6.1.0 (#2217)

Rx.NET/Source/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "6.0.2-preview.{height}",
+  "version": "6.1.0-preview.{height}",
   "publicReleaseRefSpec": [
     "^refs/heads/main$", // we release out of main
     "^refs/heads/rel/v\\d+\\.\\d+", // we also release branches starting with rel/vN.N