ESLint 9 uses the CWD as the default path.

It works fine with Node.js 20, but we might as well suggest the latest
version of Node.js.

The project is ESM now. We no longer need to use the `.mjs` extension.

This replaces a big part of the logic from `eslint-formatter-gitlab`.
This also means the generated issues now contain positions with column
data instead of just line data.