Move from Java 17 to Java 21 about freerouting HOT 2 CLOSED

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from freerouting.

In order to achieve this, I need to make sure that:

• Java 21 can be installed alongside Java 17
• the codebase compiles with Java 21
  • Fix the new issues related to URL and Locale class constructors
• upgrading Gradle to 8.6 doesn't cause problems (guide)
  • Fix the Deprecated Gradle features were used in this build, making it incompatible with Gradle 9.0. warning
  • Fix the Caused by: java.lang.module.InvalidModuleDescriptorException: Unsupported major.minor version 65.0 error
• the three GitHub Actions workflows are updated to Java version 21
• the Windows installer version runs even if no Java is installed
• the KiCad script looks for Java 21 and installs JDK 21 if it's missing
• the documentation is updated on GitHub

from freerouting.