Open new entry in changelog

As explained at[1], piuparts job setup local repository with packages
just built. If their version is equal to version in archives, job will
fail with "Sources disagree on hashes of supposely identical version"
error. So, to keep CI clean, it is necessary to create new changelog
entry after release.

 [1] salsa-ci-team/pipeline#46 (comment 80587)
8 jobs for master in 14 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #280051
build

00:03:34

 
  Test
passed #280056
autopkgtest

00:03:04

passed #280057
blhc

00:01:32

passed #280055
lintian

00:01:52

passed #280058
piuparts

00:06:52

passed #280054
reprotest

00:03:34

passed #280053
test-build-all

00:03:34

passed #280052
test-build-any

00:03:40