man: add v257 tag

Even though v256-final is not released yet, let's v257 tag now to make
not PRs for v257 conflict with each other.

(cherry picked from commit 13c8a3d)

Author: yuwata

man/version-info.xml

@@ -78,4 +78,5 @@
   <para id="v254">Added in version 254.</para>
   <para id="v255">Added in version 255.</para>
   <para id="v256">Added in version 256.</para>
+  <para id="v257">Added in version 257.</para>
 </refsect1>