Mattermost 5.34.2 Update

See package MR: https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/merge_requests/20

Just a tag bump for the image, not additional changes required.

Closes https://repo1.dso.mil/platform-one/big-bang/apps/collaboration-tools/mattermost/-/issues/7