Remove reference to 3.7 and 3.8 backports. (GH-20754)

diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst
index 69a20fc..38ad041 100644
--- a/Doc/library/decimal.rst
+++ b/Doc/library/decimal.rst
@@ -2193,4 +2193,3 @@
 .. [#]
     .. versionchanged:: 3.9
        This approach now works for all exact results except for non-integer powers.
-       Also backported to 3.7 and 3.8.