Fix Minecraft 1.21.1 build and animated mesh rendering

[kltyton]

The 1.21.1 branch fails against its mapped Minecraft and NeoForge APIs and bypasses deformed geometry when native X bend is zero. This completes the existing port for Fabric and NeoForge and keeps the deformed-mesh render path active during PAL animation.

• Match CubeDefinition.dimensions to its 1.21.1 Vector3f descriptor and use the static FML loading-list API.
• Select the Java 21 toolchain, update the matching PAL dependency to 1.1.6, and supply PAL's Javassist runtime dependency to the NeoForge development run.
• Disable the original-cube rendering path at animation render entry and restore it at exit, matching the other supported version branch.

Validation: Gradle build -Pmod_version=2.0.0+alpha.1 passes for both loaders. Fabric and NeoForge development clients were tested in the overworld. An additional Fabric downstream animation test exercised X/Y/Z bend and bend-only position/scale; its manual validation passed after checking that the running remapped dependency contained the fix. No server-side behavior or unrelated renderer integrations are changed.