Spring-ai Standardize builder class names in vector stores

• Rename all specific builder inner classes (PineconeBuilder, MongoDBBuilder, etc.) to simply Builder for consistency across vector store implementations
• Update code references to use the new standardized Builder class names

The change establishes a consistent naming convention for builder classes across the vector store implementations, improving code uniformity.

Comment From: markpollack

merged in d697e58c056aa0a3aeadd058bac8554a6f2e0736

Will can circle back on the signature for CassandraVectorStore's builder.