www/mattermost-server: Update to 8.1.12

The old hack to overwrite go.mod and replace some unsupported keywords
did not work anymore so we roll our own distfile now including
vendorized dependencies.

PR: 278462
Approved by: maintainer (implicit)

(cherry picked from commit 6b177df5fcb8048b66d24a1cab847338e6aa34ac)