Upgrade to JDK 21

[simonhir]

Is your feature request related to a problem? Please describe.

LCM to use newest JDK LTS version 21.

Currently blocked by Optimize JDK21 support coming with version 3.14 on 2024-10-08: https://docs.camunda.org/enterprise/announcement/#camunda-optimize-3-14

Describe the solution you'd like

• Upgrade Code and CI to use JDK21
• Upgrade Optimize to 3.14

Describe alternatives you've considered

• Stay at JDK 17

Acceptance Criteria

• [ ] Code can be built local with JDK21
• [ ] Code is modified to use JDK21: Maven, Docker images
• [ ] CI uses JDK21 and succeeds
• [ ] Optimize uses version 3.14 and plugins work correctly

Additional context