Use the client ID for the the iss claim also.

This switches from using the webwork server url for the iss claim to
using the client id in the JWT sent when requesting an access token from
the LMS for grade passback.

This is confirmed to work on Moodle, Canvas, and D2L.

Author: drgrice1

lib/WeBWorK/Authen/LTIAdvantage/SubmitGrade.pm

@@ -138,7 +138,7 @@ async sub get_access_token ($self) {
 		encode_jwt(
 			payload => {
 				aud => $ce->{LTI}{v1p3}{AccessTokenAUD},
-				iss => $c->url_for('root')->to_abs->to_string,
+				iss => $ce->{LTI}{v1p3}{ClientID},
 				sub => $ce->{LTI}{v1p3}{ClientID},
 				jti => $private_key->{kid}
 			},