Golang x/build/cmd/relui: add support for mentioning security fixes in Release Candidate release kind

Right now relui supports sending release announcements to the golang-announce mailing list that mention security fixes only for minor Go releases:

https://cs.opensource.google/go/x/build/+/master:internal/task/announce.go;l=358-366;drc=e76a69f0ab0814ff62f062a451d47716544df406

Release Candidates can also have security fixes (e.g., see the go1.24rc2 announcement for a recent example), and it'd be good to add support for that. Even in cases where that announcement is sent out of band, it would still help with being able to use the relui workflow for making posts to multiple social media sites (also see #71319).

CC @golang/release, @cherrymui, @golang/security.