Release debian/1.20-6

Changes:
dns-flood-detector (1.20-6) unstable; urgency=medium
.
[ Jan Wagner ]

• [9f76895] Adding d/.gitlab-ci.yml
• [2fdc34f] Bump Standards-Version to 4.5.1.0, no changes needed
  .
  [ Helmut Grohne ]
• [9db1d5f] Fix FTCBFS: Let dpkg's buildtools.mk supply $(CC).
  (Closes: #949599)
  .
  [ Jan Wagner ]
• [88f1ad9] Adding Github CI
• [17a80e0] d/control: Raise compat level to 12