migration/definition: accept named migrations
Created by: bobheadxi
Implements the first step of https://github.com/sourcegraph/sourcegraph/issues/37244:
modify readDefinitions in internal/database/migration/definition/read.go to ignore anything after the first _ in the Atoi call
Test plan
Unit tests show this works for both existing migrations and migrations that have names in them
go run ./dev sg migration leaves <commit>
works