Hilla: Outdated documentations for Parser plugins and options

[taefi]

As of 24.7, there are no more options to set, for example, exposedPackagesToParser or hilla extension object for Gradle configurations.
The documentations still pointing to those configurations should be refined / updated to reflect the new reality.

Since we had these possibilities before, it can be a good idea to mention how the current implementation is an enhancement, or to clarify whether it is a missing feature which going to be added again or not.

I could spot the following places to update, but there might be more:

• https://vaadin.com/docs/latest/hilla/reference/endpoint-generator#adding-a-custom-generator-plugin
• https://vaadin.com/docs/latest/hilla/reference/gradle#_all_options

[platosha]

Let's replace this with the mention in the upgrade guide addressing the exposedPackagesToParser.