[etherpad-lite]: Fix update instructions

[bugtrapper]

Since the 2.x version, updating an Etherpad installation via git pull often results in a broken installation and requires a fresh install.

[bugtrapper]

The Etherpad developers suggest using

git checkout <version>
git switch -c <version>

instead of

git checkout  -- src/package.json
git pull origin <version>

I don't know yet if that solves the problem or even works reliably on uberspace.

[bugtrapper]

I've tried around a bit with these update processes since 2.5.1, but neither works reliably. git checkout <version> just gives me an error (error: Pfadspezifikation 'v2.5.3' stimmt mit keinen Git bekannten Dateien überein), while git pull origin <version> breaks my etherpad instance, making it unable to be started.

So far, I've resorted to backing up my settings.json and doing a fresh install. Should we change the upgrading instructions accordingly?

@ezzra , are you possibly still running and updating an etherpad instance?