Update hardcoded upgrade tool to 23.6.3 (#4570)

Co-authored-by: Zhe Sun <31067185+ZheSun88@users.noreply.github.com>

Author: vaadin-bot

frontend/demo/upgrade-tool/upgrade-tool.ts

@@ -23,7 +23,7 @@ const VAADIN_VERSIONS: Record<string, string> = {
   20: '20.0.8',
   21: '21.0.9',
   22: '22.0.28',
-  23: '23.6.2',
+  23: '23.6.3',
 };
 
 const SIMPLE_VERSIONS = Object.keys(VAADIN_VERSIONS);