Consider publishing signatures as .sig instead of .asc

Someone posted a blog comment with this request. It doesn't seem unreasonable.

https://blog.torproject.org/comment/285450#comment-285450

added TorBrowserTeamTriaged component::applications/tor browser gitlab-tb-tor-browser-build owner::tbb-team priority::medium severity::normal status::new tbb-rbm tbb-sign type::enhancement labels

Trac:
Keywords: N/A deleted, tbb-rbm added

#31295 (moved) is somewhat related

Trac:
Keywords: N/A deleted, tbb-sign added

Add magic gitlab keyword.

Trac:
Keywords: N/A deleted, gitlab-tb-tor-browser-build added

moved to tpo/applications/tor-browser-build#32479 (closed)