Fix GKE update version check

[kahun]

What type of PR is this?
/kind bug

What this PR does / why we need it:
The GKE update process enters an infinite loop during the reconcile process because it compares desiredMasterVersion to InitialClusterVersion instead of CurrentMasterVersion.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Use current cluster version when upgrading a GKE cluster instead of the initial version. 

[k8s-ci-robot]

Hi @kahun. 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.

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[cpanato]

/ok-to-test

[richardcase]

Thanks for this @kahun 🙇 With a fix like this its helpful to add a release note instead of using NONE because this will then get highlighted when we do a release. I have updated the release note for this PR.

[richardcase]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kahun, richardcase

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[kahun]

Thanks for this @kahun 🙇 With a fix like this its helpful to add a release note instead of using NONE because this will then get highlighted when we do a release. I have updated the release note for this PR.

Thank you for the feedback; I will keep it in mind for future PRs :P