add a setter to avoid breaking Observable.return_type (#4798)

**Context:**
Some plugin tests failed because they were setting this property, which
I did not expect.

**Description of the Change:**
Add a setter to the `Observable.return_type` property to avoid breaking
things in this release. Also warn if someone uses it.

**Benefits:**
We don't break `Observable.return_type` in this release.

**Possible Drawbacks:**
People can still set it? idk seems like a safe call for now. I'm not
validating the type provided

**Related GitHub Issues:**
missed in #4762

doc/releases/changelog-dev.md

@@ -56,6 +56,7 @@
 * `Observable.return_type` is deprecated. Instead, you should inspect the type
   of the surrounding measurement process.
   [(#4762)](https://github.com/PennyLaneAI/pennylane/pull/4762)
+  [(#4798)](https://github.com/PennyLaneAI/pennylane/pull/4798)
 
 <h3>Documentation 📝</h3>
 

pennylane/operation.py

@@ -1887,6 +1887,16 @@ def return_type(self):
         )
         return self._return_type
 
+    @return_type.setter
+    def return_type(self, value):
+        """Change the return type of an Observable. Note that this property is deprecated."""
+        warnings.warn(
+            "`Observable.return_type` is deprecated. Instead, you should "
+            "create a measurement process containing this Observable.",
+            UserWarning,
+        )
+        self._return_type = value
+
     def __matmul__(self, other):
         if active_new_opmath():
             return super().__matmul__(other=other)

tests/test_operation.py

@@ -886,6 +886,11 @@ class DummyObserv(qml.operation.Observable):
         with pytest.warns(UserWarning, match="`Observable.return_type` is deprecated. Instead"):
             assert DummyObserv(0, wires=[1]).return_type is None
 
+        obs = DummyObserv(0, wires=[1])
+        with pytest.warns(UserWarning, match="`Observable.return_type` is deprecated. Instead"):
+            # pylint:disable=attribute-defined-outside-init
+            obs.return_type = qml.measurements.Sample
+
     def test_construction_with_wires_pos_arg(self):
         """Test that the wires can be given as a positional argument"""