Change priority extra to priority optional.

Changes-By: lintian-brush
Fixes: lintian: priority-extra-is-replaced-by-priority-optional
See-also: https://lintian.debian.org/tags/priority-extra-is-replaced-by-priority-optional.html

debian/control

 Source: spamdb
 Section: admin
-Priority: extra
+Priority: optional
 Maintainer: ale <ale@incal.net>
 Build-Depends: debhelper-compat (= 12),
                dh-golang,