fix approval-checking GRANDPA voting rule

[rphmeier]

a None return value implies to vote on the best, not to vote on the base.

this explicitly changes the logic to vote on the base

NOTE: DO NOT RELEASE UNTIL THOROUGHLY TESTED ON ROCOCO

[ordian]

Seems to do what the comment says now, but it would be good to extract this logic into a function and add a test for future-proofing. Also seems a bit weird that we now always return Some.

[rphmeier]

Also seems a bit weird that we now always return Some.

I agree, but that is IMO a GRANDPA API issue - the approval-checking voting rule is really opinionated, so we have to be explicit in informing the caller of what we want to vote on.

[andresilva]

We could return None when ParachainVotingRuleTarget::Current so that the voting rule doesn't filter anything. Looks good to me as is though.