[avalanche] Use MIN_VALID_PROOF_SCORE where appropriate

Summary: Bumping the dust threshold will break these hard coded values, so use MIN_VALID_PROOF_SCORE instead.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11820