Use the new onion-site.svg icon in the onion-location pill

The .onion available pill is using the old onion-glyph in 12.5a7:

It should use the new onion site icon instead, as per #41749 (closed).

I think this may have been missed during the onion.svg untangling that happened in #41809 (closed), or I caused even more confusion with this reply:

onion-site.svg should only be used in place of the security padlock in the location bar

Which was inaccurate because the original design also included the onion-location pill.