Update Timefold solver version

Author: zepfred

start-site/src/main/resources/application.yml

@@ -164,7 +164,7 @@ initializr:
         versionProperty: timefold-solver.version
         mappings:
           - compatibilityRange: "[3.3.0,3.5.0-M1)"
-            version: 1.20.1
+            version: 1.21.0
       vaadin:
         groupId: com.vaadin
         artifactId: vaadin-bom