Update release checklist

Add two items to the release checklist to ensure that the start and the
end of the code freeze for each release cycle is announced on
Mattermost.
73 jobs for !5165 with michal/update-release-checklist
latest detached
Status Job ID Name Coverage
  Autoconf
canceled autoreconf #1786386
amd64 linux

 
  Precheck
canceled clang-format #1786388
amd64 linux

canceled coccinelle #1786389
amd64 linux

canceled danger #1786390
amd64 linux

canceled misc #1786387
amd64 linux

canceled tarball-create #1786391
amd64 linux

 
  Build
canceled clang:asan #1786404
amd64 linux

canceled clang:buster:amd64 #1786408
amd64 linux

canceled clang:freebsd12:amd64 #1786411
amd64 libvirt

canceled clang:freebsd13:amd64 #1786412
amd64 libvirt

canceled clang:openbsd:amd64 #1786413
amd64 libvirt

canceled clang:tsan #1786406
amd64 linux

canceled gcc:alpine3.13:amd64 #1786392
amd64 linux

canceled gcc:asan #1786403
amd64 linux

canceled gcc:bionic:amd64 #1786401
amd64 linux

canceled gcc:buster:amd64 #1786396
amd64 linux

canceled gcc:centos7:amd64 #1786393
amd64 linux

canceled gcc:centos8:amd64 #1786394
amd64 linux

canceled gcc:focal:amd64 #1786402
amd64 linux

canceled gcc:mutexatomics #1786407
amd64 linux

canceled gcc:out-of-tree #1786398
amd64 linux

canceled gcc:sid:amd64 #1786397
amd64 linux

canceled gcc:softhsm2.4 #1786409
amd64 linux

canceled gcc:softhsm2.6 #1786410
amd64 linux

canceled gcc:stretch:amd64 #1786395
amd64 linux

canceled gcc:tarball #1786399
amd64 linux

canceled gcc:tsan #1786405
amd64 linux

canceled gcc:tumbleweed:amd64 #1786400
amd64 linux

 
  Unit
canceled unit:clang:asan #1786424
amd64 linux

canceled unit:clang:buster:amd64 #1786428
amd64 linux

canceled unit:clang:freebsd12:amd64 #1786431
amd64 libvirt

canceled unit:clang:freebsd13:amd64 #1786432
amd64 libvirt

canceled unit:clang:openbsd:amd64 #1786433
amd64 libvirt

canceled unit:clang:tsan #1786426
amd64 linux

canceled unit:gcc:alpine3.13:amd64 #1786414
amd64 linux

canceled unit:gcc:asan #1786423
amd64 linux

canceled unit:gcc:bionic:amd64 #1786421
amd64 linux

canceled unit:gcc:buster:amd64 #1786418
amd64 linux

canceled unit:gcc:centos7:amd64 #1786415
amd64 linux

canceled unit:gcc:centos8:amd64 #1786416
amd64 linux

canceled unit:gcc:focal:amd64 #1786422
amd64 linux

canceled unit:gcc:mutexatomics #1786427
amd64 linux

canceled unit:gcc:sid:amd64 #1786419
amd64 linux

canceled unit:gcc:softhsm2.4 #1786429
amd64 linux

canceled unit:gcc:softhsm2.6 #1786430
amd64 linux

canceled unit:gcc:stretch:amd64 #1786417
amd64 linux

canceled unit:gcc:tsan #1786425
amd64 linux

canceled unit:gcc:tumbleweed:amd64 #1786420
amd64 linux

 
  System
canceled system:clang:asan #1786444
amd64 linux

canceled system:clang:buster:amd64 #1786448
amd64 linux

canceled system:clang:freebsd12:amd64 #1786451
amd64 libvirt

canceled system:clang:freebsd13:amd64 #1786452
amd64 libvirt

canceled system:clang:tsan #1786446
amd64 linux

canceled system:gcc:alpine3.13:amd64 #1786434
amd64 linux

canceled system:gcc:asan #1786443
amd64 linux

canceled system:gcc:bionic:amd64 #1786441
amd64 linux

canceled system:gcc:buster:amd64 #1786438
amd64 linux

canceled system:gcc:centos7:amd64 #1786435
amd64 linux

canceled system:gcc:centos8:amd64 #1786436
amd64 linux

canceled system:gcc:focal:amd64 #1786442
amd64 linux

canceled system:gcc:mutexatomics #1786447
amd64 linux

canceled system:gcc:sid:amd64 #1786439
amd64 linux

canceled system:gcc:softhsm2.4 #1786449
amd64 linux

canceled system:gcc:softhsm2.6 #1786450
amd64 linux

canceled system:gcc:stretch:amd64 #1786437
amd64 linux

canceled system:gcc:tsan #1786445
amd64 linux

canceled system:gcc:tumbleweed:amd64 #1786440
amd64 linux

 
  Docs
canceled docs #1786453
amd64 linux

 
  Postcheck
canceled cppcheck #1786457
amd64 linux

canceled flake8 #1786454
amd64 linux

canceled gcov #1786458
amd64 linux

canceled pylint #1786455
amd64 linux

canceled scan-build #1786456
amd64 linux