fix: beacon: validate drand change at nv16 correctly

[arajasek]

Related Issues

@rvagg reported an inability to sync epoch 510 on calibnet, the UpgradeSkyrHeight (nv16). This is because that upgrade included a "bump" in drand, that required 2 rounds to be included at that epoch. For more see #2170 and #8606.

The implementation of FIP-0073 introduced a new check that block headers only include precisely those rounds as matching the block epochs. This check works correctly for all epochs EXCEPT the UpgradeSkyrHeight, but is only needed for nv22 and after when we have unchained randomness.

Proposed Changes

Apply the new check only if the source of randomness is unchained (effectively saying nv22 and after only).

Additional Info

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