Mattermost 10.6: Important upgrade notes

@plu8 Below is a note of important upgrade notes in our upcoming release.

Mattermost v10.6 (GitLab 17.11 - April, 2025)

• Support for PostgreSQL v11 and v12 have been removed. The new minimum PostgreSQL version is v13+. See the minimum supported PostgreSQL version policy documentation for details.
• Migration times: On a system with 12M posts, and 1M fileinfo entries, the migration takes 15s. This migration is non-locking. Note that there is no migration for MySQL deployments because this optimization is only applicable for PostgreSQL.