chore: start work on Timefold Solver 2.0.0

[triceo]

Description of the change

Description here

Checklist

Development

• The changes have been covered with tests, if necessary.
• You have a green build, with the exception of the flaky tests.
• UI and JS files are fully tested, the user interface works for all modules affected by your changes (e.g., solve and analyze buttons).
• The network calls work for all modules affected by your changes (e.g., solving a problem).
• The console messages are validated for all modules affected by your changes.

Code Review

• This pull request includes an explanatory title and description.
• The GitHub issue is linked.
• At least one other engineer has approved the changes.
• After PR is merged, inform the reporter.

[Copilot]

Pull request overview

This PR begins the Timefold Solver 2.0.0 upgrade work by raising the Java baseline across the quickstarts and aligning CI workflows accordingly.

Changes:

• Bump Maven compiler release (and Kotlin jvmTarget) from 17 to 21 across multiple quickstarts.
• Bump Gradle sourceCompatibility/targetCompatibility from 17 to 21 for the Gradle-based quickstarts.
• Update GitHub Actions workflows to use newer Java versions (and update Maven version in the release workflow).

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
kotlin/school-timetabling/pom.xml	Raise Java release to 21 and Kotlin jvmTarget to 21.
java/vehicle-routing/pom.xml	Raise Java release to 21.
java/tournament-scheduling/pom.xml	Raise Java release to 21.
java/task-assigning/pom.xml	Raise Java release to 21.
java/spring-boot-integration/pom.xml	Raise Java release to 21.
java/spring-boot-integration/build.gradle	Set Gradle Java compatibility to 21.
java/sports-league-scheduling/pom.xml	Raise Java release to 21.
java/school-timetabling/pom.xml	Raise Java release to 21.
java/school-timetabling/build.gradle	Set Gradle Java compatibility to 21.
java/project-job-scheduling/pom.xml	Raise Java release to 21.
java/order-picking/pom.xml	Raise Java release to 21.
java/meeting-scheduling/pom.xml	Raise Java release to 21.
java/maintenance-scheduling/pom.xml	Raise Java release to 21.
java/hello-world/pom.xml	Raise Java release to 21.
java/hello-world/build.gradle	Set Gradle Java compatibility to 21.
java/food-packaging/pom.xml	Raise Java release to 21.
java/flight-crew-scheduling/pom.xml	Raise Java release to 21.
java/facility-location/pom.xml	Raise Java release to 21.
java/employee-scheduling/pom.xml	Raise Java release to 21.
java/conference-scheduling/pom.xml	Raise Java release to 21.
java/bed-allocation/pom.xml	Raise Java release to 21.
.github/workflows/release.yml	Use Java 21 for release workflow; update Maven version used by the workflow.
.github/workflows/pull_request_maven_long_running.yml	Switch long-running jobs to run on Java 25.
.github/workflows/pull_request_maven.yml	Remove Java 17 from the Maven PR matrix (now 21 and 25).
.github/workflows/pull_request_gradle.yml	Run Gradle PR workflow on Java 21 instead of 17.

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