grandpa: Ensure `WarpProof` stays in its limits (#6963)

There was the chance that a `WarpProof` was bigger than the maximum warp
sync proof size. This could have happened when inserting the last
justification, which then may pushed the total proof size above the
maximum. The solution is simply to ensure that the last justfication
also fits into the limits.

Close: https://github.com/paritytech/polkadot-sdk/issues/6957

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Co-authored-by: command-bot <>