Proto: attestation with different slots are not slashed (under feature flag)

What

Under attestation_aggregate feature flag, double attesting evidence because of different slots are deemed invalid by the protocol.

Why

Under aggregate_attestation feature flag, attestation slots are no longer part of the signed payload when signed by a bls key. Therefore, a misbehaviour regarding the slot value cannot be attributable to the delegate.

To prevent the network of being flooded with non-minimal slots by a malicious peer (that was the risk covered by the slashing), the mempool is refusing (and therefore not propagating) attestations with non-minimal slots in !16793 (merged)

Note that the accuser is updated accordingly by no longer denouncing this behaviour in !15888 (merged)

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• 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).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR