Add xhtml deprecation notice.

xhtml is dead. Plan on removing it in next release along with the code
that supports it.

Author: rouilj

doc/upgrading.txt

@@ -76,6 +76,15 @@ upgrade all your trackers at once.
   optional packages are dropping Python 2 support. As a result Python 2
   may not be supported for many more release cycles.
 
+.. admonition:: XHTML Support Deprecation Notice
+
+  If you are running a tracker where the ``html_version`` setting in
+  ``config.ini`` is ``xhtml``, you should plan to change your
+  templates to use html (HTML5). If you are affected by this, please
+  send email to the roundup-users mailing list (roundup-users at
+  lists.sourceforge.net). Version 2.3.0 is expected to be the last
+  version to support XHTML.
+
 Contents:
 
 .. contents::

roundup/configuration.py

@@ -1044,6 +1044,8 @@ def str2value(self, value):
             'The dispatcher is configured using the DISPATCHER_EMAIL\n'
             ' setting.'),
         (Option, "html_version", "html4",
+            "This setting should be left at the default value of html4.\n"
+            "Support is ending for xhtml mode.\n"
             "HTML version to generate. The templates are html4 by default.\n"
             "If you wish to make them xhtml, then you'll need to change\n"
             "this setting to 'xhtml' too so all auto-generated HTML\n"