Update to Minecraft 1.21.6-rc1 and Kotlin 2.1.21

Minecraft 1.21.6-rc1 was released recently, so here is a PR to update the Silk Some notes: - For some reason, Mojang moved `hasPermission` method from the `SharedSuggestionProvider` interface to a new `PermissionSource` interface. And all implementations of the `SharedSuggestionProvider` also implement the `PermissionSource`, but not the `SharedSuggestionProvider` itself. So, because of it, `Source` in the command builders should also implement the `PermissionSource` now, but it shouldn't break anything. - `Component.Serializer` was removed, so I rewrote the `Component.serializeToPrettyJson` function to just use `ComponentSerialization.CODEC` and GSON.