Database version upgrades can be tricky. Always make a proper backup before! Ideally you would make a db dump plus a copy of the db container data.

Typically smaller version upgrades are not a problem, for example updating from 10.9.4 to 10.9.5 shouldnt be a issue.

But major upgrades, like going from 9.6 to 10.2 can cause more problems and you should always pay attention to the release notes of a new version and also make sure that the software that makes use of that db is also still compatible with that new db version. For example it could be that Nextcloud says to stay on 10.9 and 11.0 is not yet supported.

Since databases are usually not configured so that anyone can connect to it, let alone the open internet, it is not too bad when you lag behind a few versions. Ideally when using Docker networking for example you would have a closed network that only connects the db with the actual app that needs the app, and nothing else can connect to that db.