[ObjC] Loosen the link between generate messages and roots.

The root class only needs to be started up if the message scopes extensions.
This updates the startup flows so the root is only started when the message
scopes extensions and thus the runtime requires that startup.

Bump the generated source version to account for the new initializer that
doesn't take the rootClass argument.

Fix typo while at it.

PiperOrigin-RevId: 504899046