ref(sdk-crashes): Rename Flutter files to Dart (#71318)

Rename Flutter files to Dart, as all other files and methods are named
with Dart.

tests/sentry/utils/sdk_crashes/test_sdk_crash_detection_flutter.py → tests/sentry/utils/sdk_crashes/test_sdk_crash_detection_dart.py

@@ -4,7 +4,7 @@ from unittest.mock import patch
 
 import pytest
 
-from fixtures.sdk_crash_detection.crash_event_flutter import get_crash_event
+from fixtures.sdk_crash_detection.crash_event_dart import get_crash_event
 from sentry.testutils.helpers.options import override_options
 from sentry.testutils.pytest.fixtures import django_db_all
 from sentry.utils.safe import get_path, set_path