chore: Deprecate Scope.user (#4194)

The docstring for `Scope.user` says it's deprecated in favor of
`Scope.set_user()`, but there is no user-facing warning. Add one so that
we can [drop the
property](#4193) in the
next major.

---------

Co-authored-by: Daniel Szoke <[email protected]>

Author: sentrivana

Diff for: sentry_sdk/scope.py

@@ -794,6 +794,11 @@ def set_transaction_name(self, name, source=None):
     def user(self, value):
         # type: (Optional[Dict[str, Any]]) -> None
         """When set a specific user is bound to the scope. Deprecated in favor of set_user."""
+        warnings.warn(
+            "The `Scope.user` setter is deprecated in favor of `Scope.set_user()`.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self.set_user(value)
 
     def set_user(self, value):