Update niv-updater-action to v6

Various fixes in the output, to make it more correct, and avoid spamming
of GitHub histories. Should be faster, too.

This release brings the following:

Formatting:
* Fix: Add an empty line after the title. Now GitHub Commits list looks
  better.
* Fix: Remove preceeding space in the title
* Output branch name used by niv, so that it is clear what is being
  tracked
* Add a title prefix support via `title_prefix` option

Being nice:
* Reformat mentions in the changelog to reduce spamming of mentioned
  developers (now, @knl will not generate a GitHub notification when
  present in the changelog)
* Add option to avoid GitHub backreferencing the PR. That is, when we
  have #123 in the changelog, it will be a correct link to the correct
  repository, however, the referenced PR/issue will not contain a
  reference, thus reducing the spamming. See
  knl/niv-updater-action#26 for more details.
* Make the location of `sources.json` customizable

Performance:
* Replace the use of jo with jq, thus avoiding to install jo
* Do not install hub CLI, as it is present in the default image. This
  should avoid installing hub, speeding up the start.

.github/workflows/niv-updater.yml

@@ -18,8 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: niv-updater-action
-        # TODO: move to v6 when @knl has had time to release.
-        uses: knl/niv-updater-action@master
+        uses: knl/niv-updater-action@v6
         with:
           whitelist: 'common,advisory-db,napalm'
           title_prefix: 'build: '