[Apicurito] remove all mentions that UI image can be changed via CR

Apicurito operator description still contains parts where is mentioned that the UI image can be changed via CR.
It needs to be deleted from How to install part.
Also, part How to upgrade needs to be rewritten.

Also, a new version of OCP (4.5) contains this page:

 On the bottom of the page, there is an option to set a UI image. This should be gone too.