Update schema: public address, no schema version (#33)

* change the address to be the public one rather than a private Azure DevOps repo;
* remove the schema version, because the JSON Schema library I'm targeting for the next upgrade fails to parse the schema if I specify the version.

featureflags.schema.json

@@ -1,6 +1,5 @@
 {
-    "$schema": "http://json-schema.org/draft-07/schema#",
-    "$id": "https://onebranch.visualstudio.com/Build/_git/Powershell-FeatureFlags?path=%2Ffeatureflags.schema.json",
+    "$id": "https://raw.githubusercontent.com/microsoft/PowerShell-FeatureFlags/main/featureflags.schema.json",
     "title": "Feature Flags Schema",
     "description": "Describes a schema for the Powershell-FeatureFlags configuration file.",
     "type": "object",
@@ -107,4 +106,4 @@
             }
         }
     }
-}
+}