gh-115119: defer removal of bundled libmpdec to 3.16 (configure changes)

Author: skirpichev

configure

@@ -4134,13 +4134,13 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       [have_mpdec=no])
   ])],
   [AC_MSG_WARN([m4_normalize([
-     the bundled copy of libmpdecimal is scheduled for removal in Python 3.15;
+     the bundled copy of libmpdecimal is scheduled for removal in Python 3.16;
      consider using a system installed mpdecimal library.])])])
 
 AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
       [AC_MSG_WARN([m4_normalize([
          no system libmpdecimal found; falling back to bundled libmpdecimal
-         (deprecated and scheduled for removal in Python 3.15)])])
+         (deprecated and scheduled for removal in Python 3.16)])])
        USE_BUNDLED_LIBMPDEC()])
 
 # Disable forced inlining in debug builds, see GH-94847