[release] Update release config (flutter#49254)

Updates automated release note configuration with the following:

* Corrects the label name for impeller changes.
* Adds fallback "Other Changes" section that includes unlabeled changes.

Author: itsjustkevin

.github/release.yml

@@ -6,7 +6,7 @@ changelog:
   categories:
     - title: Impeller
       labels:
-        - impeller
+        - "e: impeller"
     - title: Android
       labels:
         - platform-android
@@ -20,3 +20,6 @@ changelog:
       labels:
         - platform-windows
         - platform-macos
+    - title: Other Changes
+      labels:
+        - "*"