Add release notes for 1.12.1, 1.12.2 and 1.12.3

[inteon]

These release notes were missing.
Based on https://github.com/cert-manager/cert-manager/releases

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[maelvls]

Thanks for updating the change log.

/lgtm

nit: the latest released version should come on top since that's what people should be looking at when they arrive on the 1.12 page: they should be aware of what the earliest version on the 1.12 branch is. This is what Kubernetes does: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.27.md

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This pull-request has been approved by: inteon, maelvls

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[maelvls]

/lgtm