chore: archive ethereum/execution-spec-tests post-weld

[danceratopz]

We still need to decide what "archiving" eest entails. The repo will still be used for release tarballs and maybe re-purposed to publish test releases as JSON as source, so that client teams can git submodule it in their repos.

• "delete" source? Just add a note to the readme guiding people to execution-specs and explaining that client teams will still find release tarballs here?
• close all PRs
• ensure no open issues: handled by chore: move remaining issues to execution-specs execution-spec-tests#2201

[marioevz]

Modify README.md to point newcomers to EELS.

[marioevz]

We might need to keep main branch frozen for a while until we know that most teams that could be using the code (as opposed to the releases of the fixtures) have properly migrated over to EELS.

This will require probably hand-holding and reaching out via Eth R&D to see who could need help with this.

TL;DR:

• main stays as is, frozen with the exception of README.md updates that help teams transition to EELS.
• In parallel we can create forks/osaka, forks/amsterdam branches that contain the filled fixtures from EELS.