Document Postgres update (major to major)

[TheFrenchGhosty]

Follow up to https://github.com/iv-org/documentation/pull/200#discussion_r811677892

Postgres update (between major versions) requires dumping (from the old version) and importing the dump (to the new version).

We should document it, ideally we should have copy-paste ready (for docker and non-docker install)

[unixfox]

Related: https://github.com/docker-library/postgres/issues/37