refactor: Unify all json.(loads|dumps) usage to utils.json (#28702)

Co-authored-by: Eyal Ezer <eyal.ezer@ge.com>

tests/integration_tests/annotation_layers/api_tests.py

@@ -17,14 +17,13 @@
 # isort:skip_file
 """Unit tests for Superset"""
 
-import json
-
 import pytest
 import prison
 from sqlalchemy.sql import func
 
 import tests.integration_tests.test_app  # noqa: F401
 from superset import db
+from superset.utils import json
 from superset.models.annotations import Annotation, AnnotationLayer
 
 from tests.integration_tests.base_tests import SupersetTestCase