mattermost update to 9.7.3-bb.0

Package Merge Request

Package Changes

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/9.7.3-bb.0/CHANGELOG.md

Package MR

big-bang/product/packages/mattermost!207 (merged)

For Issue

Closes big-bang/product/packages/mattermost#116 (closed)

Upgrade Notices

N/A