move `Kubeflow on AWS` to legacy distributions

[thesuperzapper]

As indicated by AWS in this thread https://github.com/awslabs/kubeflow-manifests/issues/794, the Kubeflow on AWS distribution is no longer actively maintained.

This PR updates the "installing kubeflow" table to reflect this fact.

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[thesuperzapper]

@surajkota if you can confirm the Kubeflow on AWS distribution is no longer maintained then @zijianjoy can you please merge this PR.

This is important to avoid confusion for new users who are using AWS, to prevent them from adopting an inactive distribution.

[thesuperzapper]

@jbottum @zijianjoy I have separated the style changes that were in this PR into a separate one https://github.com/kubeflow/website/pull/3643 (which I think we should merge, as they help new users).

This PR is not only for moving AWS to legacy, so will allow us to give AWS more time to confirm if they plan to continue supporting their distribution.

[jbottum]

@thesuperzapper I think we need to consider that the last AWS release was for 1.7 and has a date of Sept 23 per this link, https://github.com/awslabs/kubeflow-manifests/releases, which I believe is within the 6 month window that you have defined as an active distribution.

[andreyvelich]

/hold due to https://github.com/kubeflow/website/pull/3693