Document new DOMjudge patch release

Author: vmcj

website/header.shtml

@@ -1,5 +1,5 @@
-<!--#set    var="dj_current_branch_version"  value="8.1.2"
---><!--#set var="dj_current_branch_date"     value="23 July 2022"
+<!--#set    var="dj_current_branch_version"  value="8.1.3"
+--><!--#set var="dj_current_branch_date"     value="19 September 2022"
 --><!--#set var="dj_previous_branch_version" value="8.0.1"
 --><!--#set var="dj_previous_branch_date"    value="11 July 2022"
 --><!--#set var="dj_rc_branch_version"       value="6.0.0RC1"