chore: Simplify license attribution setup for entire project

Instead of relying on individual license attribution per file, configure license attribution in the REUSE.toml file. Moreover, use the generic "Signstar Contributors" as attribution and in a comment point out that the license attribution is not tracked individually, but that interested parties can get to this information with the help of git.

Signed-off-by: David Runge dvzrv@archlinux.org