Updated mattermost to 7.7.1-bb.0

Package Merge Request

Package Changes

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/7.7.1-bb.0/CHANGELOG.md

Package MR

big-bang/product/packages/mattermost!109 (merged)

For Issue

Closes big-bang/product/packages/mattermost#61 (closed)