Spring-ai Consolidate google vertex starters into one starter

There are separate boot starters for chat and embedding. To be consistent in the starters across the project, these two starters should be merged into a single boot starter for google vertex.

Comment From: markpollack

from

<dependency>
    <groupId>org.springframework.ai</groupId>
    <artifactId>spring-ai-vertex-ai-embedding-spring-boot-starter</artifactId>
</dependency>

and

<dependency>
    <groupId>org.springframework.ai</groupId>
    <artifactId>spring-ai-vertex-ai-gemini-spring-boot-starter</artifactId>
</dependency>

to

<dependency>
    <groupId>org.springframework.ai</groupId>
    <artifactId>spring-ai-vertex-ai-spring-boot-starter</artifactId>
</dependency>

which would autoconfigure both the embedding and chat models. We can deprecate the current starters in M2 and remove them in RC.