Declare compliance with policy 4.1.5, bump debhelper compat level to 11

debian/compat

@@ -1 +1 @@
-10
+11

debian/control

@@ -1,10 +1,10 @@
 Source: tang
 Maintainer: Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
 Homepage: https://github.com/latchset/tang
-Standards-Version: 4.1.1
+Standards-Version: 4.1.5
 Vcs-Browser: https://git.in-ulm.de/cbiedl/tang
 Vcs-Git: https://git.in-ulm.de/cbiedl/tang.git
-Build-Depends: debhelper (>= 10~),
+Build-Depends: debhelper (>= 11~),
     curl,
     jose,
     libhttp-parser-dev (>= 2.8.0),