Bump minimum Nix version to 2.18 (#2779)

## Summary

Nixpkgs now requires Nix >= 2.18 to evaluate, so drop support for older
versions. This updates the CI test matrices and MinVersion constant.

## How was it tested?

Run in CI to confirm Nix Version checks are running correctly

## Community Contribution License

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>