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 #1786386
amd64 linux
autoreconf

 
  Precheck
canceled #1786388
amd64 linux
clang-format

canceled #1786389
amd64 linux
coccinelle

canceled #1786390
amd64 linux
danger

canceled #1786387
amd64 linux
misc

canceled #1786391
amd64 linux
tarball-create

 
  Build
canceled #1786404
amd64 linux
clang:asan

canceled #1786408
amd64 linux
clang:buster:amd64

canceled #1786411
amd64 libvirt
clang:freebsd12:amd64

canceled #1786412
amd64 libvirt
clang:freebsd13:amd64

canceled #1786413
amd64 libvirt
clang:openbsd:amd64

canceled #1786406
amd64 linux
clang:tsan

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

canceled #1786403
amd64 linux
gcc:asan

canceled #1786401
amd64 linux
gcc:bionic:amd64

canceled #1786396
amd64 linux
gcc:buster:amd64

canceled #1786393
amd64 linux
gcc:centos7:amd64

canceled #1786394
amd64 linux
gcc:centos8:amd64

canceled #1786402
amd64 linux
gcc:focal:amd64

canceled #1786407
amd64 linux
gcc:mutexatomics

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

canceled #1786397
amd64 linux
gcc:sid:amd64

canceled #1786409
amd64 linux
gcc:softhsm2.4

canceled #1786410
amd64 linux
gcc:softhsm2.6

canceled #1786395
amd64 linux
gcc:stretch:amd64

canceled #1786399
amd64 linux
gcc:tarball

canceled #1786405
amd64 linux
gcc:tsan

canceled #1786400
amd64 linux
gcc:tumbleweed:amd64

 
  Unit
canceled #1786424
amd64 linux
unit:clang:asan

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

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

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

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

canceled #1786426
amd64 linux
unit:clang:tsan

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

canceled #1786423
amd64 linux
unit:gcc:asan

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

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

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

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

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

canceled #1786427
amd64 linux
unit:gcc:mutexatomics

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

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

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

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

canceled #1786425
amd64 linux
unit:gcc:tsan

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

 
  System
canceled #1786444
amd64 linux
system:clang:asan

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

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

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

canceled #1786446
amd64 linux
system:clang:tsan

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

canceled #1786443
amd64 linux
system:gcc:asan

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

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

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

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

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

canceled #1786447
amd64 linux
system:gcc:mutexatomics

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

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

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

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

canceled #1786445
amd64 linux
system:gcc:tsan

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

 
  Docs
canceled #1786453
amd64 linux
docs

 
  Postcheck
canceled #1786457
amd64 linux
cppcheck

canceled #1786454
amd64 linux
flake8

canceled #1786458
amd64 linux
gcov

canceled #1786455
amd64 linux
pylint

canceled #1786456
amd64 linux
scan-build