Breaking Change: Update major version numbers for C++ and Python.

This is per the breaking change announcement in https://protobuf.dev/news/2024-10-02/

Note that Objective-C's major version number was bumped already in cl/684131809

PiperOrigin-RevId: 686247631

version.json

@@ -4,13 +4,13 @@
         "lts": false,
         "date": "2024-09-30",
         "languages": {
-            "cpp": "5.30-dev",
+            "cpp": "6.30-dev",
             "csharp": "3.30-dev",
             "java": "4.30-dev",
             "javascript": "3.30-dev",
             "objectivec": "4.30-dev",
             "php": "4.30-dev",
-            "python": "5.30-dev",
+            "python": "6.30-dev",
             "ruby": "4.30-dev"
         }
     }