flatcar: Set cloud-provider kubelet flag to "external"

[johananl]

In-tree cloud provider support was removed in k8s v1.27.

See the following:

• CHANGELOG-1.27: Add note for AWS in-tree provider removal kubernetes/kubernetes#118899
• Remove AWS legacy cloud provider + EBS in-tree storage plugin kubernetes/kubernetes#115838
• fix: the quickstart guide needs update for external cloud provider #4301

What type of PR is this?

/kind bug

What this PR does / why we need it:

Support for in-tree cloud providers was removed, which is currently causing errors when using templates/cluster-template-flatcar.yaml. Set cloud-provider to external to fix this.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

None

Special notes for your reviewer:

• I'm not sure why in some places it's OK to use cloud-provider: aws and in others it's breaking things. See fix: the quickstart guide needs update for external cloud provider #4301 (comment).
• I'm not sure if I need to update the e2e tests.

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

NONE

[Ankitasw]

@johananl there is one E2E test for flatcar which already uses cloud-provider: external
Thanks for fixing this.
This is just the document change(templates) which was missed when the cloud provider migration was done.
So I think there is no other file for flatcar, this change should be ok.

[Ankitasw]

/lgtm
/approve

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[johananl]

Thanks @Ankitasw!

This is just the document change(templates) which was missed when the cloud provider migration was done.

IIUC this is more than a document change since the template files are used by clusterctl generate to produce workload cluster manifests (that's how I noticed the problem - my generated cluster wouldn't boot).