proofs: stop verifying reproducibility of old prestate builds

The `build-prestates.sh` script builds ALL old prestates for every op-program tag, and for some kona tags from the old [op-rs/kona](https://github.com/op-rs/kona) repo.

We could limit how far back we go:
1. Drop support for building prestates from the op-rs/kona repo (remove `build_legacy_kona_prestate()`)
2. Use the `--contains` flag on `git tag` to only return tags later than a cutoff.