Bump debhelper dependency to >= 13, since that's what is used in debian/compat.

Changes-By: lintian-brush
Fixes: lintian: package-lacks-versioned-build-depends-on-debhelper
See-also: https://lintian.debian.org/tags/package-lacks-versioned-build-depends-on-debhelper.html
Fixes: lintian: package-needs-versioned-debhelper-build-depends
See-also: https://lintian.debian.org/tags/package-needs-versioned-debhelper-build-depends.html