Update Mattermost to 6.3.4

Package Merge Request

Package Changes

Update MM to 6.3.4

Package MR

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/85

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/issues/44 (sort of)