Add flatcar-stable to supported OS list for clusterawsadm

[sedefsavas]

What this PR does / why we need it:
To automatically list pre-built flatcar-stable in the book, added it to supported OS list.

Also, images can be listed with clusterawsadm.
clusterawsadm ami list --kubernetes-version=1.23.3 --os=flatcar-stable

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #3107

Special notes for your reviewer:

Checklist:

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

Release note:

Add flatcar-stable os to supported OS list, will publish reference AMIs with each new K8s release.

[k8s-ci-robot]

@sedefsavas: This issue is currently awaiting triage.

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[shivi28]

We can add flatcar-stable OS at following places 1 2 3 4 also.
/lgtm

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[k8s-ci-robot]

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