chore(renovate): don't age-gate submodule digest bumps

git-submodules digest updates carry no releaseTimestamp, and Renovate's
default minimumReleaseAgeBehaviour (timestamp-required) treats that as
never old enough, so the protocol submodule bump sat in "Pending Status
Checks" forever and no PR was ever opened.

Author: u9g

renovate.json

@@ -38,6 +38,10 @@
       "matchManagers": ["git-submodules"],
       "matchDepNames": ["livekit-rtc/rust-sdks"],
       "enabled": false
+    },
+    {
+      "matchManagers": ["git-submodules"],
+      "minimumReleaseAge": null
     }
   ]
 }