packaging: stop using dpkg-sig

The dpkg-sig tool appears to no longer be maintained and, as far as I
can tell, was never actually used by Debian or Ubuntu to verify the
integrity of packages. Both distributions no longer include the dpkg-sig
package in their newest releases [1, 2].

[1] https://tracker.debian.org/pkg/dpkg-sig
[2] https://manpages.ubuntu.com/manpages/jammy/en/man1/dpkg-sig.1.html

Change-Id: I356ff7eafb340a49622cc37899dad7ef84dbd100