core[minor]: Update pgvector transalor for langchain_postgres (#23217)

The SelfQuery PGVectorTranslator is not correct. The operator is "eq"
and not "$eq".
This patch use a new version of PGVectorTranslator from
langchain_postgres.

It's necessary to release a new version of langchain_postgres (see
[here](langchain-ai/langchain-postgres#75)
before accepting this PR in langchain.

Author: pprados

libs/langchain/langchain/retrievers/self_query/base.py

@@ -170,11 +170,12 @@ def _get_builtin_translator(vectorstore: VectorStore) -> Visitor:
 
         try:
             from langchain_postgres import PGVector
+            from langchain_postgres import PGVectorTranslator as NewPGVectorTranslator
         except ImportError:
             pass
         else:
             if isinstance(vectorstore, PGVector):
-                return PGVectorTranslator()
+                return NewPGVectorTranslator()
 
         raise ValueError(
             f"Self query retriever with Vector Store type {vectorstore.__class__}"