Backport rename of service-account key to 1.20

[johngmyers]

Addresses #11370

[justinsb]

Like #11390, this looks good, but I propose we get an upgrade e2e periodic test in asap

/hold

[hakman]

@justinsb should be covered by @rifelpet's new upgrade test:
https://testgrid.k8s.io/kops-misc#kops-aws-kops-upgrade

I see this in logs, which I am not sure if it's the expected change:

  Keypair/service-account
  	Signer              	 id:cn=service-account -> <nil>
  	LegacyFormat        	 true -> false

[johngmyers]

@hakman The LegacyFormat change is saying that it will upgrade the format (by copying it from the "master" slot to "service-account". For the "Signer" field, nil means "self-signed" so it is not an actual change.

[johngmyers]

For an upgrade from 1.18 or earlier, the signer would change from "ca" to being self-signed.

[justinsb]

Reproducing #11390 is non-trivial (which is a good thing, as it means this isn't as critical as it first appears)!

/hold cancel

/approve
/lgtm

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