Upgrade to Mattermost 10.11

[gsanchietti]

Current Mattermost version is 10.5.14 and it's now EOL.

Upgrade to latest 10.11.x release:
https://docs.mattermost.com/product-overview/mattermost-v10-changelog.html#release-v10-11-extended-support-release

The 10.11 release will be EOL on 08/2026