[Elasticsearch] BM25 retrieval is too restrictive

[anakin87]

To Reproduce

from elasticsearch_haystack.bm25_retriever import ElasticsearchBM25Retriever
from elasticsearch_haystack.document_store import ElasticsearchDocumentStore

document_store = ElasticsearchDocumentStore(hosts= "http://localhost:9200/")

documents = [Document(content="There are over 7,000 languages spoken around the world today."),
  Document(content="Elephants have been observed to behave in a way that indicates a high level of self-awareness, such as recognizing themselves in mirrors."),
  Document(content="In certain parts of the world, like the Maldives, Puerto Rico, and San Diego, you can witness the phenomenon of bioluminescent waves.")]

retriever = ElasticsearchBM25Retriever(document_store=document_store)
print(retriever.run(query="How much self awareness do elephants have?"))
# {'documents': []}

This should return the 2nd Document but this does not happen because of this AND operator:

haystack-core-integrations/integrations/elasticsearch/src/elasticsearch_haystack/document_store.py

Line 266 in 65beef5

"operator": "AND",

See for comparison the same query in Haystack 1.x:
https://github.com/deepset-ai/haystack/blob/c812250453ab7da35f526a5f2a53e18c058fe2ff/haystack/document_stores/search_engine.py#L1100