Merge and alias against_vehicle tags

BUR #21145 has been rejected.

create alias leaning_against_vehicle -> against_vehicle
create alias leaning_against_motorcycle -> against_vehicle

These two tags basically describe a similar concept. All against_* tags already indicate that a character's posture is leaning on/against some kind of inanimate object.
Added leaning against motorcycle into this BUR because we do not use far more specific variants like this for other vehicles like "leaning_against_boat".

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 #21145 (forum #259736) has been rejected by @DanbooruBot.

Leaning against a car and a motorcycle are significantly different.