Fix milk clay bucket not having capability

[VoidLeech]

All of FluidClayBucketWrapper already had support for the milk bucket, it just wasn't initialised. Looks like it was removed from the base bucket class in 2c8a4918c9381fcdb5128abe1d569050dfca4ad9 and mistakenly not added back to the milk one. Result was that it could be filled by capability, but not drained.