[avalanche] Leverage AnyVoteItem to have a single VoteItemUpdate object

Summary:
Remove the type specific vote update, so a single VoteItemUpdate can be used and remove some boilerplate.

There is no change in behavior.

Depends on D12995, D12996, D12997.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12998