Spring-ai Remove PaLM Model support as it is deprecated and will be decomissioned

See https://ai.google.dev/palm_docs/deprecation

The API will be decommissioned in Auguest 15th, so there is no point in keeping it in Spring AI as we will be post 1.0 GA by then.

Comment From: ddobrin

PaLM2 support will end in Oct 2024, and can be removed https://ai.google.dev/palm_docs/palm

Comment From: markpollack

remember to remove documentation as well.