Add empty post-update hooks to clear all caches when updating to 4.0.0

Problem/Motivation

If you have Automatic Updates 3.x installed, it will be using its own bundled version of Package Manager. If you then update to Automatic Updates 4, which uses core's Package Manager, you are liable to encounter a WSOD until you at least clear all caches.

Proposed resolution

Add an empty post-update hook to both Automatic Updates and Automatic Updates Extensions, to force a container rebuild and prevent WSODs (hopefully).