Update Mattermost to 6.4.1

Package Merge Request

Package Changes

Updated to latest 6.4.1 image for MM.

Package MR

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/86

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/issues/46