Mattermost 9.1: Important upgrade notes

@twk3 Below is a note of important upgrade notes in our upcoming release.

Mattermost v9.1 (GitLab 16.6 - November 22)

• Improved performance on data retention DeleteOrphanedRows queries. See the Important Upgrade Notes for notes on a new migration that was added. Removed feature flag DataRetentionConcurrencyEnabled. Data retention now runs without concurrency in order to avoid any performance degradation. Added a new configuration setting DataRetentionSettings.RetentionIdsBatchSize, which allows admins to to configure how many batches of IDs will be fetched at a time when deleting orphaned reactions. The default value is 100.
• Minimum supported Mattermost Desktop App version is now v5.3. OAuth/SAML flows were modified to include desktop_login which makes earlier versions incompatible.