meta: Add deprecation warning for js globals in custom plugins (#26425)

This adds a deprecation warning to our changelog regarding custom plugins:

The frontend bundle will be loaded asynchronously. This is a breaking change that can affect custom plugins that access certain globals in the django template. Please see https://forum.sentry.io/t/breaking-frontend-changes-for-custom-plugins/14184 for more information

CHANGES

@@ -1,3 +1,10 @@
+Unreleased
+----------
+
+### Deprecation Warning
+
+There will be a future change to where the frontend bundle will be loaded asynchronously. This will be a breaking change that can affect custom plugins that access certain globals in the django template. Please see https://forum.sentry.io/t/breaking-frontend-changes-for-custom-plugins/14184 for more information.
+
 21.5.1
 ------