Rename one_knee

BUR #12049 has been rejected.

create alias one_knee -> one_knee_down

Current tag is very ambiguous.

BUR #12563 has been rejected.

create alias one_knee -> kneeling_on_one_leg

Submitting this as a separate BUR since the previous one is almost a month old.

This might be a better name, it's consistent with kneeling and is the original name of the tag before the rename in topic #13837.

This bulk update request is pending automatic rejection in 5 days.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #12049 (forum #220356) has been rejected by @DanbooruBot.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #12563 (forum #222620) has been rejected by @DanbooruBot.