Parallelize `rake gettext:lint` to improve runtime

What does this MR do and why?

This MR improves the overall runtime of rake gettext:lint by parallelizing linting utilizing the already used gem parallel. It also removes an unused method html_todolist.

On my machine, this changes cuts the runtime from ~55s to ~25s 🚀

It has also positive impact on runtime in CI:

Before	After
==> Finished in 100.554665931 seconds (expected 85 seconds)	==> Finished in 77.605701638 seconds (expected 85 seconds)

Additionally, users now see a bar showing the progress of the lint:

Linting po files | ETA: 00:00:12 | =============                                                   | Time: 00:00:03

See 👇

Screenshots or screen recordings

Before	After

How to set up and validate locally

Run rake gettext:lint and enjoy the progress bar.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.