fix(@schematics/angular): canLoad guard signature

add UrlTree to canLoad guard signature to match the documentation and the other implementations

Closes: #18751

Author: geromegrignon

packages/schematics/angular/guard/files/__name@dasherize__.guard.ts.template

@@ -25,7 +25,7 @@ export class <%= classify(name) %>Guard implements <%= implementations %> {
   }
   <% } %><% if (implements.includes('CanLoad')) { %>canLoad(
     route: Route,
-    segments: UrlSegment[]): Observable<boolean> | Promise<boolean> | boolean {
+    segments: UrlSegment[]): Observable<boolean | UrlTree> | Promise<boolean | UrlTree> | boolean | UrlTree {
     return true;
   }<% } %>
 }