Migrate project to Java 21

Summary

Migrate the project compilation target from Java 16 to Java 21.

Is this feature is related to a specific Minecraft Version ? NO

Feature Description

Update all pom.xml files to use Java 21 as the compilation target. This includes:

• Root pom.xml
• core/pom.xml
• All NMS module pom.xml files (API, v1_19_R3, v1_20_R1, v1_20_R2, v1_20_R3)

Also fix deprecated reflection API usage in NMSManager.java.

Motivation and Benefits

• Java 21 is an LTS version with better performance and new features
• Paper 1.19.4+ servers commonly run on Java 21
• Avoid deprecation warnings and potential future incompatibilities

Use Cases

• Running the plugin on modern Paper servers using Java 21
• Using new Java 21 language features in future development

Additional Context

All supported NMS versions (1.19.4, 1.20.1, 1.20.2, 1.20.4) are compatible with Java 21.

Possible Implementation

• Update maven.compiler.source and maven.compiler.target to 21 in all pom.xml
• Replace deprecated Class.newInstance() with getDeclaredConstructor().newInstance()