mattermost update to 11.9.0-bb.6

Package Merge Request

Package Changes

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/11.9.0-bb.6/CHANGELOG.md

Package MR

big-bang/product/packages/mattermost!380 (merged)

For Issue

Closes big-bang/product/packages/mattermost#287 (closed)

Upgrade Notices

N/A