Proto/AI: prevent a potential double slashing at protocol migration

What

Fix a potential double slashing of a double signing event on a round 1+ during the last cycle of Oxford, that was introduced in !11826 (merged).

How

The problem is that the round of misbehaviors denounced during Oxford was not saved, so we set their round to zero during stitching to P. So for misbehaviors that happened during Oxford, we check for a duplicate in the Already_denounced table not only with their actual round, but also with round zero.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [N/A (no deployed version had this problem)] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).