Add Ubuntu impish

See merge request !21

Release 2.13.1-1

See merge request !18

Note: The release builds for this security update were prepared in a private
fork before publishing the release and were uploaded from there:
https://git.icinga.com/packaging/icinga-2-security-releases-2.13.1-2.12.6-2.11.11/deb-icinga2

Drop Ubuntu groovy

See merge request !13

Add Debian Bullseye

See merge request !7

Add Ubuntu Hirsute

See merge request !8

Drop Ubuntu Xenial

See merge request !9

Drop Ubuntu eoan

See merge request !6

.gitlab-ci.yml: drop Debian "jessie"

See merge request !4

Ubuntu 20.04 LTS