Update Mattermost to 5.3.4

chart/values.yaml

@@ -1044,7 +1044,7 @@ addons:
     git:
       repo: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost.git
       path: "./chart"
-      tag: "0.5.0-bb.0"
+      tag: "0.6.0-bb.0"
 
     # -- Flux reconciliation overrides specifically for the Mattermost Package
     flux: {}