Update project to Java 21 and Spring Boot 4.0.2

[mark79-github]

• Update dependency versions in pom.xml.
• Update GitHub Actions workflow to use JDK 21.
• Update Dockerfile to use the Java 21 alpine image.
• Set system properties to require Java 21.
• Replace WithMockUser annotations with MockMvc user setup.
• Use Authentication instead of Principal in HomeController.
• Restrict /log/** endpoint to ROOT role only.
• Remove unnecessary Async and EnableAsync annotations.
• Fix incorrect annotations in test files to match Spring Boot 4 changes.