Reduce boilerplate in GH action worklows

[brb]

Currently, we have tons of boilerplate in our GH action workflows (e.g., steps are not reused, same tests but different workflows for different Cilium versions, etc). Due to the boilerplate it's easy to make a copy-paste mistake when editing or creating new workflows.

We could either use methods listed in https://github.blog/2022-02-10-using-reusable-workflows-github-actions/ or use some templating for the workflow YAML files.