feat(mod): Support 1.21.6

[3arthqu4ke]

No description provided.

[3arthqu4ke]

This requires a change in headlessmc-lwjgl to fix a division by zero:

at java.base/java.lang.Math.floorDiv(Math.java:1472) ~[?:?] {}
	at TRANSFORMER/minecraft@1.21.6/net.minecraft.util.Mth.positiveCeilDiv(Mth.java:659) ~[client-1.21.6-20250617.151856-srg.jar%23204!/:?] {re:classloading}
	at TRANSFORMER/minecraft@1.21.6/net.minecraft.util.Mth.roundToward(Mth.java:655) ~[client-1.21.6-20250617.151856-srg.jar%23204!/:?] {re:classloading}
	at TRANSFORMER/minecraft@1.21.6/net.minecraft.client.renderer.DynamicUniformStorage.<init>(DynamicUniformStorage.java:31) ~[client-1.21.6-20250617.151856-srg.jar%23204!/:?] {re:classloading,pl:runtimedistcleaner:A}

[okafke]

Currently blocked because we cannot update to 1.21.10, because unimined does not work for that version

[okafke]

Also needs headlesshq/headlessmc#334
And headlesshq/headlessmc#335