Pin microshift-bootc image for openshift-4.21 4.21.16

[openshift-art-build-bot]

Created by job run https://art-jenkins.apps.prod-stable-spoke1-dc-iad2.itup.redhat.com/job/aos-cd-builds/job/build%252Fbuild-microshift-bootc/309

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[openshift-ci]

@openshift-art-build-bot[bot]: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/art-pre-commit-check	c3fcc9b	link	false	/test art-pre-commit-check

Full PR test history. Your PR dashboard.

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