Change default value of middleware_spans to False (#15852)

https://github.com/getsentry/sentry-python/releases/tag/2.48.0

The newest release sets this to False.

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Author: pinguin999

docs/platforms/python/integrations/django/index.mdx

@@ -153,7 +153,7 @@ You can pass the following keyword arguments to `DjangoIntegration()`:
 
   Create spans and track performance of all middleware in your Django project. Set to `False` to disable.
 
-  The default is `True`.
+  The default is `False`.
 
 - `signals_spans`: