DOCS: Updated AWS Management Cluster docs

[cdenneen]

/kind feature

https://github.com/kubernetes-sigs/cluster-api-provider-aws/blob/master/docs/book/src/topics/using-cluster-api-with-cross-account-role-assumption.md

These instructions still mention clusterctl alpha phases which is no longer viable. These docs need to be updated.

[richardcase]

/help
/good-first-issue
/kind documentation

[k8s-ci-robot]

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[Evalle]

I'm happy to contribute.
/assign

[Evalle]

I got sidetracked, don't want to hold this to be implemented, feel free to re-assign it.

[richardcase]

@Evalle - we've added a milestone of 0.7.0 to this, so there is plenty of time if you would still like to do this?

[fejta-bot]

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[richardcase]

/lifecycle frozen