Remove unnecessary greater-than versioned dependency to dpkg-dev, libbrotli-dev, libapache2-mod-md // Declare compliance with policy 4.2.1

Closes: #880993

Fixes lintian: insecure-copyright-format-uri
See https://lintian.debian.org/tags/insecure-copyright-format-uri.html for more details.

Fixes lintian: file-contains-trailing-whitespace
See https://lintian.debian.org/tags/file-contains-trailing-whitespace.html for more details.

Fixes lintian: debian-control-has-obsolete-dbg-package
See https://lintian.debian.org/tags/debian-control-has-obsolete-dbg-package.html for more details.

Otherwise the setenvif test fails in the test suite.

It turns out the test suite refuses to run as root but returns true even
in that case. Fix this by creating a temp user.

It seems this has been broken since 2.4.27-4, where the test suite had
been updated and the the debci test duration dropped from 15min to 3min.
Also, don't rely on the exit status anymore but parse the test output.

Needs to be in postrm/preinst, too

sneaked in accidentally