Query extension: standard `neq` operator is missing

[mcucchi9]

Issue

The Query extension STAC API specification states that the "not equal" operator should be available as neq, but the QueryExtension class in stac-fastapi-pgstac defines it as ne (here).

Proposed solution

Implement the neq operator alongside the ne operator (for backward compatiblity). The implementation should be analogous to the implementations of the other operators, as pgstac provides a neq operator having the same effect of the ne operator.