Merge branch 'update-mattermost-tag-10.4.2-bb.0' into 'master'

mattermost update to 10.4.2-bb.0

See merge request !5683

(cherry picked from commit a8be6c0f)

15b45fc1 Updated mattermost git tag

Co-authored-by: Michael Martin <michaelmartin@seed-innovations.com>