Execute modernization plan: Upgrade to Java 21 and Spring Boot 3.4#14

Copilot wants to merge 7 commits into

upgrade-to-lts-20260212111023qianwens/PhotoAlbum-Java:upgrade-to-lts-20260212111023from

copilot/execute-upgrade-plan-againqianwens/PhotoAlbum-Java:copilot/execute-upgrade-plan-againCopy head branch name to clipboard