attribute 'signing' is not declared for element 'SingleLogoutService'

[egobrc at gmail.com]

Hi,
I am migrating my previous Shibboleth 2.0 SP to a 3.0 version.
Moreover, my IDP asked to enable signing of Single logout messages, so
I added 'signing="true"' to 'md:SingleLogoutService' :

Before:
 <md:SingleLogoutService Location="/SLO/Redirect"
conf:template="bindingTemplate.html"
Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect"/>

After:
 <md:SingleLogoutService Location="/SLO/Redirect"
conf:template="bindingTemplate.html"
Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect"
signing="true"/>

But it generates an XML error that prevents shibboleth daemon
starting. Here is what I found in log file:

2021-04-14 09:09:08 ERROR XMLTooling.ParserPool : error on line 123,
column 172, message: attribute 'signing' is not declared for element
'SingleLogoutService'
2021-04-14 09:09:08 ERROR Shibboleth.Config : error while loading
resource (/etc/shibboleth/shibboleth2.xml): XML error(s) during
parsing, check log for specifics
2021-04-14 09:09:08 FATAL Shibboleth.Config : caught exception while
loading configuration: XML error(s) during parsing, check log for
specifics

As far as I understand from Shibboleth 3.0 documentation, signing
should be compatible with SingleLogoutService:
https://wiki.shibboleth.net/confluence/display/SP3/SingleLogoutService

Maybe it is an issue related to SP2 to SP3 migration? Here are my
shibboleth2.xml starting lines:
<?xml version="1.0"?>
<SPConfig xmlns="urn:mace:shibboleth:3.0:native:sp:config"
xmlns:conf="urn:mace:shibboleth:3.0:native:sp:config"
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata" clockSkew="180">

Regards