✨ Add availableUpgrades for ROSAMachinePool and update doc

[serngawy]

What type of PR is this?
Add availableUpgrades for ROSAMachinePool.Status and update ROSA upgrade doc.

What this PR does / why we need it:

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 #

Special notes for your reviewer:

Checklist:

• [ X] squashed commits
• [ X] includes documentation
• includes emojis
• adds unit tests
• adds or updates e2e tests

Release note:

Update ROSA upgrade documentation 

[k8s-ci-robot]

Hi @serngawy. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[serngawy]

@nrb @AndiDog would you add ok-to-test

[nrb]

/ok-to-test

[serngawy]

Would you @nrb @AndiDog review & merge if there is no comments

[nrb]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nrb

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Needs approval from an approver in each of these files:

• config/crd/bases/OWNERS [nrb]
• docs/book/src/topics/rosa/OWNERS [nrb]
• exp/api/v1beta2/OWNERS [nrb]
• exp/controllers/OWNERS [nrb]

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[serngawy]

/test pull-cluster-api-provider-aws-test

[nrb]

/test pull-cluster-api-provider-aws-test

Maximum number of VPCs reached