Build with Java 25

[SirYwell]

Overview

Description

Changes needed to build with Java 25, as preparation for Minecraft 26.1.

For now, we compile using sourceCompatibility = "21" to still support Java 21.
If we bump this, we need to remove adapters ~<=1.21.4 (needs verification, but tiny-remapper fails otherwise, and class rewriting during plugin rewriting might fail too).

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.