Pin ACK runtime to v0.51.0

[knottnt]

Issue #, if available:

Description of changes:

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[michaelhtm]

/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knottnt, michaelhtm

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[knottnt]

/retest

[ack-prow]

@knottnt: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
verify-attribution	9a346a1	link	false	/test verify-attribution
s3-olm-test	9a346a1	link	false	/test s3-olm-test

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[knottnt]

/test eks-controller-test