feat: add prune workflow

[JTaeuber]

Motivation

This pull request implements a new github workflow to prune any unnecessary images and charts to make it easier to manage.

It will prune all untagged images or images that start with commit or sha256 in the py-kube-downscaler package that are older than 7 days.

It will always keep the dev tag and shouldn't prune any of our releases.

Changes

Added github workflow prune.yml.

Tests done

I've tested the workflow in dry-run in a fork.
https://github.com/JTaeuber/py-kube-downscaler/actions/runs/9188408222

TODO

• I've assigned myself to this PR

[github-actions]

File	Coverage
All files	97%	✅
test_autoscale_resource.py	99%	✅
test_cmd.py	100%	✅
test_events.py	100%	✅
test_grace_period.py	100%	✅
test_ignore_if_labels_dont_match.py	100%	✅
test_main.py	100%	✅
test_pods_force_uptime.py	100%	✅
test_resources.py	100%	✅
test_scaler.py	95%	✅
test_time.py	100%	✅

Minimum allowed coverage is 70%

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