Troubleshoot and fix Fleet-in-Rancher CI workflows

[weyfonk]

The following CI workflows have been failing every day for a while:

• Test Fleet in Rancher (example)
  • At the time of writing, this fails at step Set up latest Rancher, which could well be caused by an outdated default charts_branch value, leading to the webhook chart version pinned in the latest Rancher not being found
  • Once that is fixed, provisioning tests failures may well appear, and should be fixed
• E2E Upgrade Fleet in Rancher to HEAD (example)