mattermost update to 9.9.0-bb.0

Package Merge Request

Package Changes

https://repo1.dso.mil/big-bang/product/packages/mattermost/-/blob/9.9.0-bb.0/CHANGELOG.md

Package MR

big-bang/product/packages/mattermost!233 (merged)

For Issue

Closes (link to issue here)

Upgrade Notices

These items were removed in the upstream mattermost release, be advised if you are using any of these features

• Removed support for selfserve purchases of Mattermost Subscriptions in various flows, throughout Cloud and Self Hosted environments.
• Removed support for selfserve true up review submission in the System Console.
• Removed the PostPriority feature flag.
• Removed SelfHostedPurchase setting from config.json