Release 2.12.6-1

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
24 jobs for 2.12 in 3 minutes and 14 seconds (queued for 1 minute and 1 second)
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