build_suggestions: bump 4.21 minimum upgrade version to 4.20.12

[haircommander]

so upgrades go through https://github.com/openshift/machine-config-operator/pull/5516

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[haircommander]

context: in cri-o 1.33, a change https://github.com/cri-o/cri-o/pull/8962 was made to the default limits set for CRI-O. Now, the ulimit nofile is set much lower, with space to set it higher. however, some workloads don't expect this change, and fail (see https://issues.redhat.com/browse/OCPBUGS-62095)

This was worked around temporarily in https://github.com/openshift/machine-config-operator/pull/5308, but that workaround was not intendd to be carried in to 4.21.

Instead, we should drop-in an ignition file on upgrades from 4.20 to 4.21 to make sure existing clusters don't get this change, but new clusters started in 4.21 do.

Now we need to make sure upgrades go through that PR. it missed 4.20.11 by a day, so I feel confident it will land in 4.20.12

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