Mattermost 10.8: Important upgrade notes

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

Mattermost v10.8 (GitLab 18.1 - June, 2025)

• New tables AccessControlPolicies and AccessControlPolicyHistory will be created. The migration is fully backwards-compatible, non-locking, and zero downtime is expected.
• Support for legacy SKUs E10 and E20 licenses was removed. If you need assistance, talk to a Mattermost expert.