Spring-ai GH-924 Deprecate PaLM API support

• As a follow up to decommission the PaLM API, the PaLM API support is currently deprecated.
• Add @Deprecated notes to the model and vector store classes
• Removed the documentation entries

Resolves #GH-924

Comment From: markpollack

the classe/code can be deleted as the API endpoint doesn't exist.

Comment From: sobychacko

Merged via https://github.com/spring-projects/spring-ai/commit/d44a6aff7c5d8541b8e51af21b92fd310b16e9a6