Remove DatabaseFeatures.is_mongodb_6_3

[timgraham]

Follow up to ce390e8.

[timgraham]

@WaVEV do you remember what we had before this?

# An existing collection must be used on MongoDB 6, otherwise
# the operation will not error when unsupported.
self.connection.get_collection("django_migrations").list_search_indexes()

Was it simply __null rather than django_migrations?

[WaVEV]

@WaVEV do you remember what we had before this?

# An existing collection must be used on MongoDB 6, otherwise
# the operation will not error when unsupported.
self.connection.get_collection("django_migrations").list_search_indexes()

Was it simply __null rather than django_migrations?

The __null was for the index query, for Constraint.validate():
SELECT 1 WHERE EXISTS(subquery checking if a constraint is violated)
Maybe we need to erase this $facet 😬 also, it could be done with $documents at the beginning of the pipeline.

In this case: it makes a query over an always existing collection. So the __null idea wasn't here

[timgraham]

I think we had __null as in the last commit, otherwise why did we have a comment about MongoDB 6 requiring an existing collection?