get_integration_source: Print warning to stderr instead of stdout

scripts/get_integration_sources.py

 #!/usr/bin/env python3
 import argparse
 import re
+import sys
 
 from gitlab import Gitlab, GitlabGetError
 from gitlab.v4.objects import Group
@@ -48,7 +49,8 @@ def get_integration_sources(manifest_project: str, manifest_branch: str, group:
                 pass
             elif e.response_code == 403:  # forbidden
                 print(
-                    f"Warning: No permissions to the shared project {project.path_with_namespace}. Skipping it."
+                    f"Warning: No permissions to the shared project {project.path_with_namespace}. Skipping it.",
+                    file=sys.stderr,
                 )
             else:
                 raise