Add information about upgrading PostgresSQL version in docker container

[madalenapmelo-kp]

Based on feedback from @violoncelloCH on issue #4434:

Regarding this issue, I still think it would make sense to have a quick note about the required Postgres version and a reference to https://community.penpot.app/t/how-to-upgrade-postgresql-version-in-docker-container/2104 either in the documentation in the upgrade to v2 section or in the changelog. Thanks for considering this :)