bin: allow per cluster upgrade prepare

[anders-elastisys]

What kind of PR is this?

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Platform Administrator notice

This PR changes the number of arguments required to run the upgrade command, as with this PR you need to specify the specific cluster, sc, wc, or both to apply to both clusters as the command did previously. The ck8s-kubespray usage has been updated to clarify this:

➜ ./bin/ck8s-kubespray         
COMMANDS:
  ...
  upgrade                                     prepares config for upgrade
      args: <wc|sc|both> <version> prepare

What does this PR do / why do we need this PR?

This PR allows for running the upgrade prepare command each cluster (wc and sc) separately, which is particularly useful for multi-workload cluster environments, or environments without workload clusters. Previously the upgrade command always applied for both wc and sc.

• Fixes #315

Additional information to reviewers

Screenshots

Checklist

• [x] Proper commit message prefix on all commits
• Change checks:
  • [ ] The change is transparent
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  • [ ] The change requires migration steps
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  • [ ] The metrics names did change (Grafana dashboards and Prometheus alerts were fixed)
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  • [x] The logs do not show any errors after the change
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