Remove `AttributeKeyConsensusHeight` attribute from `update_client` event

[crodriguezvega]

With 02-client refactor AttributeKeyConsensusHeight was deprecated in favour of AttributeKeyConsensusHeights in update_client event.

• Remove the attribute from the event.

The attribute key should remain, since it is still used in create_client event.

Adding needs_discussion label to discuss in what release we should remove this.