Update java version to 21

[campos20]

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[Copilot]

Pull Request Overview

This PR updates the project to use Java 21 while transitioning from the deprecated javax.validation to jakarta.validation packages. Key changes include:

• Migration of javax.validation imports to jakarta.validation in multiple controller and configuration files.
• Updates to GitHub Actions workflows for building, testing, and deploying using JDK 21 with updated action versions.
• Small stylistic improvements in client components and API endpoint concatenations.

Reviewed Changes

Copilot reviewed 43 out of 51 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
server/src/main/java/org/worldcubeassociation/statistics/controller/SumOfRanksController.java	Updated validation import to jakarta.validation for Java 21 compatibility.
server/src/main/java/org/worldcubeassociation/statistics/controller/StatisticsController.java	Updated validation import to jakarta.validation.
server/src/main/java/org/worldcubeassociation/statistics/controller/DatabaseQueryController.java	Updated validation import to jakarta.validation.
server/src/main/java/org/worldcubeassociation/statistics/controller/BestEverRanksController.java	Updated validation import to jakarta.validation.
server/src/main/java/org/worldcubeassociation/statistics/config/Swagger2Config.java	Reformatted license declaration without changing functionality.
server/src/main/java/org/worldcubeassociation/statistics/config/GlobalException.java	Updated validation exception imports to jakarta.validation.
server/src/main/java/org/worldcubeassociation/statistics/StatisticsApplication.java	Switched to jakarta.annotation.PostConstruct and removed the old javax version.
client/src/main/components/Footer.tsx	Simplified the Contact link formatting.
client/src/main/api/DatabaseQueryApi.ts	Changed base URL concatenation to include explicit slashes for endpoint paths.
.github/workflows/backtest.yaml	Configured working-directory, updated cache and JDK version to 21 along with minor path adjustments.
.github/workflows/backdeploy.yaml	Updated checkout, cache, and JDK setup to align with the JDK 21 migration and adjusted file paths.

Files not reviewed (8)

• server/Dockerfile: Language not supported
• server/build.gradle: Language not supported
• server/build.gradle.kts: Language not supported
• server/gradle/wrapper/gradle-wrapper.properties: Language not supported
• server/gradlew: Language not supported
• server/gradlew.bat: Language not supported
• server/settings.gradle: Language not supported
• server/settings.gradle.kts: Language not supported