Use pre-releases for gz-sim8 to test gz-transport 13.2.0

Signed-off-by: Ian Chen <ichen@openrobotics.org>

Author: iche033

plugins/config/repository.yaml

@@ -130,6 +130,13 @@ projects:
             type: stable
           - name: osrf
             type: nightly
+    # Use pre-releases for gz-sim for testing gz-transport 13.2.0
+    - name: gz-sim8
+      repositories:
+          - name: osrf
+            type: stable
+          - name: osrf
+            type: prerelease
     # generic regexp
     - name: gazebo.*
       repositories: