make vm-build-openbsd tries to download nonexistent 7.2 install ISO: need to update to 7.4

tests/vm/openbsd has:

    link = "https://cdn.openbsd.org/pub/OpenBSD/7.2/amd64/install72.iso"

This URL no longer works. http://cdn.openbsd.org/pub/OpenBSD/ has 7.3, 7.4 and 7.5 subdirectories, suggesting that we've fallen off the end of their support. The most recent release is 7.4 (I guess 7.5 is the currently-under-development one) so we should update to that.

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