remove weakref methods that return borrowed references

newsfragments/4528.removed.md

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+* Removed the `get_object_borrowed`, `upgrade_borrowed`, `upgrade_borrowed_as`,
+`upgrade_borrowed_as_unchecked`, `upgrade_borrowed_as_exact` methods of
+`PyWeakref` and `PyWeakrefProxy`. These returned borrowed references to weakly
+referenced data, and in principle if the GIL is released the last strong
+reference could be released, allowing a possible use-after-free error. If you
+are using these functions, you should change to the equivalent function that
+returns a `Bound<'py, T>` reference.