query_main.h: fix build error due to Filter being renamed to FieldFilter.

This fixes the following build error in `ubuntu-latest-Release-x64-static`:

```
firestore/src/main/query_main.h:185:34: error: no type named 'Operator' in 'firebase::firestore::core::Filter'
  using Operator = core::Filter::Operator;
                   ~~~~~~~~~~~~~~^
```

e.g. https://github.com/firebase/firebase-cpp-sdk/runs/6065960625

Author: dconeybe

firestore/src/main/query_main.h

@@ -182,7 +182,7 @@ class QueryInternal {
     kEndAt,
   };
 
-  using Operator = core::Filter::Operator;
+  using Operator = core::FieldFilter::Operator;
 
   Query Where(const FieldPath& field,
               Operator op,