chore: Add deprecation changelog snippet

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2326>

changes/changelog.d/postgres_env.deprecation

@@ -0,0 +1,3 @@
+The automatically generated 'DATABASE_URL' configuration in the docker setup is deprecated, please
+configure either the 'DATABASE_URL' environment variable or the 'DATABASE_HOST', 'DATABASE_USER' and
+'DATABASE_PASSWORD' environment variables instead.