feat: add values for priorityClassName

[pkalemba]

What does your PR do?

Adds priorityClassName to pods

Checklist

For all Pull Requests

• Commits are signed off
• Commits have semantic messages
• Documentation updated
• Passes ct linting

For releasing ONLY

• Chart.yaml version bumped
• CHANGELOG.md updated

[pkalemba]

@thesuperzapper can You look at it please?

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[pkalemba]

Can someone look at it ?

[thesuperzapper]

@pkalemba you are amazing! Thanks for this PR!

I have left a small number of comments that need to be fixed, but otherwise it looks good for the next release.

[thesuperzapper]

Please do not change this, as it will trigger a release of the chart if merged.

[thesuperzapper]

To keep things consistent let's use the same formatting as the other docstrings:

  ## default priorityClassName for airflow Pods (is overridden by pod-specific values)
  ## - docs for priorityClassName:
  ##   https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/
  ##
  defaultPriorityClassName: ""

[thesuperzapper]

Let's add a link for people who don't know what priorityClassName is:

  ## the priorityClassName for the db-migrations Pods
  ## - docs for priorityClassName:
  ##   https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/
  ##
  priorityClassName: ""

NOTE: we will also need to add it to all the other priorityClassName docstrings in this values file