Explicitly set update_policy on GCE instance group managers

[flopib]

Fixes #17382.

[k8s-ci-robot]

Hi @flopib. Thanks for your PR.

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[hakman]

/ok-to-test
/cc @justinsb
/assign @justinsb

[hakman]

/retest

[flopib]

Hi, would it be possible to review this shortly? Our customer is still susceptible of being affected by the linked issue.

[k8s-ci-robot]

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