build: add system check to warn of pending devstack settings removal

Note: I had originally implemented this as a `warnings.warn()` call
directly in lms/envs/devstack.py and cms/envs/devstack.py, but for
whatever reason, those warnings were getting swallowed. System checks
display more prominently, anyway.

Part of: openedx/public-engineering#247

openedx/core/djangoapps/util/apps.py

@@ -18,4 +18,5 @@ def ready(self):
         """
         Registers signal handlers at startup.
         """
-        import openedx.core.djangoapps.util.signals  # lint-amnesty, pylint: disable=unused-import, unused-variable
+        import openedx.core.djangoapps.util.signals  # pylint: disable=unused-import, unused-variable
+        import openedx.core.djangoapps.util.checks  # pylint: disable=unused-import, unused-variable

openedx/core/djangoapps/util/checks.py

@@ -0,0 +1,36 @@
+"""
+Miscellaneous system checks
+"""
+from django.conf import settings
+from django.core import checks
+
+
+_DEVSTACK_SETTINGS_MODULES = [
+    "lms.envs.devstack",
+    "lms.envs.devstack_docker",
+    "lms.envs.devstack_optimized",
+    "lms.envs.devstack_with_worker",
+    "cms.envs.devstack",
+    "cms.envs.devstack_docker",
+    "cms.envs.devstack_optimized",
+    "cms.envs.devstack_with_worker",
+]
+
+
+@checks.register(checks.Tags.compatibility)
+def warn_if_devstack_settings(**kwargs):
+    """
+    Raises a warning if we're using any Devstack settings file.
+    """
+    if settings.SETTINGS_MODULE in _DEVSTACK_SETTINGS_MODULES:
+        return [
+            checks.Warning(
+                "Open edX Devstack is deprecated, so the Django settings module you are using "
+                f"({settings.SETTINGS_MODULE}) will be removed from openedx/edx-platform by October 2024. "
+                "Please either migrate off of Devstack, or modify your Devstack fork to work with an externally-"
+                "managed Django settings file. "
+                "For details and discussion, see: https://github.com/openedx/public-engineering/issues/247.",
+                id="openedx.core.djangoapps.util.W247",
+            ),
+        ]
+    return []