Mattermost: Update cypress test image

chart/values.yaml

@@ -1276,7 +1276,7 @@ addons:
     git:
       repo: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost.git
       path: "./chart"
-      tag: "7.5.1-bb.3"
+      tag: "7.5.1-bb.4"
 
     # -- Flux reconciliation overrides specifically for the Mattermost Package
     flux: {}