Add documentation for using-final-decorator-in-unsupported-version

doc/data/messages/u/using-final-decorator-in-unsupported-version/bad.py

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+from typing import final
+
+
+@final  # [using-final-decorator-in-unsupported-version]
+class Playtypus(Animal):
+    @final  # [using-final-decorator-in-unsupported-version]
+    def lay_egg(self):
+        ...

doc/data/messages/u/using-final-decorator-in-unsupported-version/details.rst

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+The message is emitted when the ``final`` decorator is used with a Python version less than 3.8.
+The ``final`` decorator was introduced in Python version 3.8.

doc/data/messages/u/using-final-decorator-in-unsupported-version/good.py

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+class Playtypus(Animal):
+    def lay_egg(self):
+        ...

doc/data/messages/u/using-final-decorator-in-unsupported-version/related.rst

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+- `PEP 591 <https://peps.python.org/pep-0591/#the-final-decorator>`_