Status page: report migration progress as % of "PRs merged" instead of "PRs opened"

[jaimergp]

Originally posted by @h-vetinari in #2137 (comment)

One other point that I think is confusing: the progress percentage of a migration should be "PRs merged", not "PRs opened". As an extreme example, the ruby migration looks almost done (14/16 opened), but only 2(!) have been merged. That migration has separate problems, but the point is that having a PR open says nothing about whether the migration is close to finished (aside perhaps from the POV of the bot, but that's not what users/maintainers are most interested in).