Bump debhelper from old 12 to 13.

Changes-By: lintian-brush
Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

debian/control

@@ -2,7 +2,7 @@ Source: dovecot-exporter
 Section: admin
 Priority: optional
 Maintainer: Autistici/Inventati <debian@autistici.org>
-Build-Depends: debhelper-compat (= 12), golang-any (>= 1.11), dh-golang
+Build-Depends: debhelper-compat (= 13), golang-any (>= 1.11), dh-golang
 Standards-Version: 3.9.6
 
 Package: dovecot-exporter