Consider using a shorter shortname

[tidoust]

As we're looking into adding the draft to browser-specs, Specref, et al., it occurred to us that the spec's shortname is more a long name than a short one. I'm not aware of any requirement that restricts the length of a shortname but I'm wondering whether a shorter shortname could be adopted for this spec, such as secure-x-site-cookies or security-semantics-x-site-cookies.

(For background and a comparison with the longest spec shortnames in browser-specs, w3c/browser-specs#1569 (comment))