fix(promtail): prefer image.tag for app.kubernetes.io/version

[yurrriq]

trafficstars

For example, when I set image.tag=2.9.8, I'd like the resulting annotation to be app.kubernetes.io/version: 2.9.8 instead of the current app.kubernets.io/version: 2.9.3 which uses the .Chart.AppVersion. I've copied from the grafana chart, which already has the correct behaviour.

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[yurrriq]

I've changed the default image.tag from null to "" which solves the linting issue, at least locally.

[yurrriq]

And now I've updated README.md :facepalm: Hopefully this time the linter is finally happy.