Remove legacy formats from migrate-to-eslint instruction

[ericcornelissen]

Relates to #726, #846

Summary

In #846 the Migrate to ESLint instructions were updated to include instructions for the new flat config from ESLint v9. I kept the old instructions for the legacy config format around because ESLint v9 is still pretty new so it's reasonable to expect people to still be using the legacy format. At some point however, the instructions for the legacy format should be removed as there's no need to maintain those instructions indefinitely.