nightly-2024-06-18: chore: Use the elaborator by default (#5246)

# Description

## Problem\*

## Summary\*

Removes the `--use-elaborator` flag, replacing it with `--use-legacy` to
enable the old name resolver & type checker passes which are now off by
default in favor of the elaborator.

## Additional Context

No user visible changes are expected from this PR.

## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.