Migrate remaining get_platform in config to async_get_platform (#112469)

This was the only remaining case where blocking I/O might have happened
in config. It was unlikely though as async_get_component should have
pre-imported the config platform

homeassistant/config.py

@@ -1098,7 +1098,9 @@ async def merge_packages_config(
                 continue
 
             try:
-                config_platform: ModuleType | None = integration.get_platform("config")
+                config_platform: ModuleType | None = (
+                    await integration.async_get_platform("config")
+                )
                 # Test if config platform has a config validator
                 if not hasattr(config_platform, "async_validate_config"):
                     config_platform = None