gh-115119: Defer --with-system-libmpdec removal to 3.16 (#139318)

Author: sobolevn

Doc/using/configure.rst

@@ -869,9 +869,9 @@ Libraries options
    .. versionchanged:: 3.13
       Default to using the installed ``mpdecimal`` library.
 
-   .. deprecated-removed:: 3.13 3.15
+   .. deprecated-removed:: 3.13 3.16
       A copy of the ``mpdecimal`` library sources will no longer be distributed
-      with Python 3.15.
+      with Python 3.16.
 
    .. seealso:: :option:`LIBMPDEC_CFLAGS` and :option:`LIBMPDEC_LIBS`.