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: float-debug-proxy
 Section: net
-Priority: extra
+Priority: optional
 Maintainer: Autistici/Inventati <debian@autistici.org>
 Build-Depends: debhelper-compat (= 12), golang-any (>= 1.11), dh-golang
 Standards-Version: 3.9.6