Update filter extenion json schema in client.py

to be compliant with:
https://docs.ogc.org/is/19-079r2/19-079r2.html#queryables

Author: j-musial

stac_fastapi/extensions/stac_fastapi/extensions/core/filter/client.py

@@ -23,7 +23,7 @@ async def get_queryables(
         https://github.com/radiantearth/stac-api-spec/tree/master/fragments/filter#queryables
         """
         return {
-            "$schema": "https://json-schema.org/draft/2019-09/schema",
+            "$schema": "https://json-schema.org/draft/2020-12/schema",
             "$id": "https://example.org/queryables",
             "type": "object",
             "title": "Queryables for Example STAC API",