Extract the copyrigh years of a XEP from its revision

In #1300 (comment) it
was said that we do not longer simply bump the copyright year
range. But keeping the copyright year at 2021 of all XEPs, including
newer ones from 2023, is also far from ideal.

So let's simply extract the copyright year from the revision
information we already have.

xep.xsl

@@ -257,7 +257,10 @@ content: "XEP-<xsl:value-of select='/xep/header/number'/>";
             </ul></dd>
           </xsl:if>
           <dt>Copyright</dt>
-          <dd>&#169; 1999 &#x2013; 2021 XMPP Standards Foundation. <a href='#appendix-legal'>SEE LEGAL NOTICES</a>.</dd>
+          <!-- Extract the copyright years from the dates via
+               substring(). If we ever use XSL 2.0, then we could use
+               year-from-date(xs:date()) -->
+          <dd>&#169; <xsl:value-of select='substring(/xep/header/revision[position()=last()]/date,1,4)'/> &#x2013; <xsl:value-of select='substring(/xep/header/revision[position()=1]/date,1,4)'/> XMPP Standards Foundation. <a href='#appendix-legal'>SEE LEGAL NOTICES</a>.</dd>
           <dt>Status</dt>
           <dd>
             <p><xsl:choose>