Mattermost 5.33: Important upgrade notes

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

Mattermost v5.33 (GitLab 13.11 - April 22)

• Deleting a reaction is now a soft delete in the Reactions table. A schema update is required and may take up to 15 seconds on first run with large data sets.
• WebSocket handshakes done with HTTP version lower than 1.1 will result in a warning, and the server will transparently upgrade the version to 1.1 to comply with the WebSocket RFC. This is done to work around incorrect Nginx (and other proxy) configs that do not set the proxy_http_version directive to 1.1. This facility will be removed in a future Mattermost version and it is strongly recommended to fix the proxy configuration to correctly use the WebSocket protocol.