feat: ✨ move chart to subdir charts

[DrPsychick]

Why?

• To avoid an issue with chart-testing using chart-dirs=., moved the chart to subdir ./charts and adjusted the workflows etc. accordingly.
• Configure dependabot for GitHub actions
• removed unnecessary symlink to ci/helm-lint-values.yaml (removes warnings)

see chart-dirs='.' has issues: sapcc/helm-charts#7480

[fastlorenzo]

oh boy, moving the charts folder will require some testing 😄 thanks for the first PR, I'll try it out

[DrPsychick]

To be fair, the part with separating the workflow jobs could be completely reverted to reduce scope. I'm not sure if it is needed.

As for the charts directory, I had some issues with running ct lint and I stumbled over the related issue and since the default chart-dirs is charts (besides, it makes the workflow a bit simpler using checkout with defaults), I think it cannot hurt to do this change.
At latest when mailu-charts contains more than one chart it would be beneficial anyway, IMHO.

I guess mostly the publishing part needs another review/check if it is still working.