www/mattermost-{webapp,server}: Update to 5.21.0

Mattermost v5.21.0 contains low level security fixes.
Full Changlog: https://docs.mattermost.com/administration/changelog.html#release-v5-21-feature-release

PR: 244136
Reported by: Marius Halden <marius.halden@modirum.com>
Approved by: loic.blot@unix-experience.fr (maintainer)
MFH: 2020Q3
Sponsored by: Netzkommune GmbH