Consolidate google vertex starters into one starter

[markpollack]

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.

[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.