Merge branch 'update-mattermost-tag-7.8.0-bb.0' into 'master'

mattermost update to 7.8.0-bb.0

Closes big-bang/product/packages/mattermost#63

See merge request !2556

chart/values.yaml

@@ -1250,7 +1250,7 @@ addons:
     git:
       repo: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost.git
       path: "./chart"
-      tag: "7.7.1-bb.0"
+      tag: "7.8.0-bb.0"
 
     # -- Flux reconciliation overrides specifically for the Mattermost Package
     flux: {}