mattermost update to 9.7.3-bb.2

Package Merge Request

Package Changes

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/9.7.3-bb.2/CHANGELOG.md

Package MR

big-bang/product/packages/mattermost!193 (merged)

For Issue

Closes big-bang/product/packages/mattermost#109 (closed)

Upgrade Notices

N/A