GitHub Actions: Test mixed-version clusters against RabbitMQ 4.0.4

instead of 4.0.3.

[Why]
We need a bugfix in the Khepri reset code backported in #12739 and
publihed in RabbitMQ 4.0.4.

Author: dumbbell

.github/workflows/test-make-target.yaml

@@ -57,7 +57,7 @@ jobs:
       uses: dsaltares/fetch-gh-release-asset@master
       if: inputs.mixed_clusters
       with:
-        version: 'tags/v4.0.3'
+        version: 'tags/v4.0.4'
         regex: true
         file: "rabbitmq-server-generic-unix-\\d.+\\.tar\\.xz"
         target: ./