Update to Slimefun Experimental version (1.21) and fix gui title

[eygwi]

Description

This PR updates the addon to support Minecraft 1.21 and the latest Slimefun (Experimental) builds, while also fixing a bug with the machine GUI titles.

Changes included:

• Minecraft 1.21 Support: Updated pom.xml dependencies and repositories.
• Java 21: Updated maven.yml (GitHub Actions) to use JDK 21 and actions/checkout@v4 to ensure successful builds on the new target version.
• API Updates: Refactored code to align with the latest Slimefun API changes (Experimental builds).
• Bug Fix: Fixed an issue in ResinExtractor.java where the Inventory Title and Machine Identifier were hardcoded.
  • Before: All tiers showed "Resin Extractor" and shared the same ID.
  • After: Tiers now correctly display their names (e.g., "Resin Extractor (II)") and use unique IDs (e.g., RESIN_EXTRACTOR_2).

Related Issues

Resolves #24

Testing:

• The project now builds successfully with Java 21.
• Verified that machine GUIs display the correct tier names.