Consider standard naming conventions for vector dbs

I'm not sure we'll ever get them aligned as we have been building them based on what is most sensible for a free user (which in some cases is also most sensible for a paid user) who wants multiple Search API Indexes (in the Search API sense of the word).

Pinecone free tier (serverless) :
- Index = Search API Server https://docs.pinecone.io/guides/manage-data/manage-indexes
- Namespace = Search API Index https://docs.pinecone.io/guides/manage-data/manage-namespaces

Milvus/Zilliz https://milvus.io/docs/manage-collections.md :
- Database = Search API Server https://milvus.io/docs/manage_databases.md
- Collection = Search API Index https://milvus.io/docs/manage-collections.md

I think when @seogow (if I remember right) converted the first to be a VDB provider, it was with Milvus/Zilliz which is why the language in AiVdbProviderInterface is *Collection.

Maybe lining up with Search API terminology and ignoring the conflicting terms of the providers themselves might be less confusing for users. What do you think?

IMO it would make sense to stick with the Search API language already well established with the Drupal community.