Bump transitive dependency flatted from 3.4.1 to 3.4.2 (#3424)

## Summary

- Upgrades the transitive dependency `flatted` from 3.4.1 to 3.4.2 to
resolve [Dependabot security alert
#265](https://github.com/WordPress/wordpress-playground/security/dependabot/265)
- `flatted` is pulled in transitively via `eslint` → `flat-cache` and
`@nx/react` → `log4js`, both of which use semver ranges (`^3.2.x`) that
already allow 3.4.2
- Only the lockfile resolution changed — no direct dependency upgrades
needed

## Test plan

- [x] Verify `npm ls flatted` shows 3.4.2
- [ ] Verify CI passes (no functional changes, only lockfile update)


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Author: ashfame