Update Mattermost to 5.38.2

Package Owner Merge Request

Package Changes

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/58

https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/tags/0.2.0-bb.0

Straightforward image update + updates to Cypress test due to the UI changes

Additional Details

Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/701