Change: Portal target removed

BREAKING CHANGE for developers: The (undocumented) portal target `app.runtime.header` is not available anymore. Please use the extension point `app.runtime.header.center` with `customComponent` extensions instead (for details see below).

The portal target `app.runtime.header` has been removed in favour of a new extension point with the id `app.runtime.header.center`. The extension point
is capable of mounting extensions of type `customComponent`. The search bar, which was previously using this portal target, was rewired into an extension. Other `portal` instances which used this portal target won't work anymore
and need to be ported to the `customComponent` extension type instead.

https://github.com/owncloud/web/pull/10758
