Remove reference to 3.7 and 3.8 backports. (GH-20754) (#20755)

(cherry picked from commit 3231883)

Authored-by: Stefan Krah <skrah@bytereef.org>

Author: miss-islington

Doc/library/decimal.rst

@@ -2193,4 +2193,3 @@ are expected to be exact.
 .. [#]
     .. versionchanged:: 3.9
        This approach now works for all exact results except for non-integer powers.
-       Also backported to 3.7 and 3.8.