chore(lint): eslint config comment inverts the "type": "module" rule, and ecmaVersion 2021 caps flat config's default

Follow-up from the review of #240 (merged), which migrated eslint 8 -> 9 and added a flat config. Two items in eslint.config.mjs, neither affecting current behaviour.

1. The comment inverts the rule it states

eslint.config.mjs currently opens with:

// Flat-config port of the previous .eslintrc. The .mjs extension is required:
// this package is "type": "module".

That is backwards. package.json does declare "type": "module", and that is precisely the case where the .mjs rename is not required — a plain eslint.config.js is already parsed as ESM, so eslint loads it as-is. .mjs is what a CommonJS package needs, in order to write an ESM config in a package whose .js files are CJS.

The reviewer verified this empirically: renaming the file to eslint.config.js and running eslint loaded it fine.

Fix: either rename to eslint.config.js and delete the comment, or keep .mjs (harmless, and eslint resolves it) and correct the comment to say it is a stylistic choice rather than a requirement. Leaving the inverted claim in place will mislead whoever ports the next repo by copying this file.

2. ecmaVersion: 2021 is now the only thing capping the parser

languageOptions: {
  ecmaVersion: 2021,
  sourceType: 'module',
  ...
}

2021 was carried over faithfully from the old .eslintrc, which is the right default for a migration PR — it preserves behaviour. But flat config defaults ecmaVersion to latest, so under .eslintrc the cap was one setting among the file's inherited defaults, and now it is an explicit and deliberate-looking restriction. Nothing in the repo needs post-2021 syntax today, so this is not breaking anything; it just means the next person to use, say, Array.prototype.at chaining or newer syntax hits a parse error for no reason.

Fix: drop the ecmaVersion line and let it default to latest. globals.es2021 in the same block can stay or move to globals.es2025 independently — it controls globals, not syntax.

References

• Merged PR: https://github.com/nearform/node-test-parser/pull/240
• Review raising both points: https://github.com/nearform/node-test-parser/pull/240#pullrequestreview-4958809517

Start with eslint.config.mjs, then check package.json to confirm the package module type. Correct the misleading .mjs comment or apply the suggested rename, remove the explicit ecmaVersion: 2021 setting, and run ESLint to confirm the configuration still loads successfully.