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

_Originally posted by @h-vetinari in https://github.com/conda-forge/conda-forge.github.io/issues/2137#issuecomment-2303047226_

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](https://github.com/conda-forge/conda-forge-pinning-feedstock/issues/4805), 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).