Fix: gettext:po_to_json deprecation issue

What does this MR do and why?

• Address the locale compilation issue introduced in !115042 (merged).
• Update documentation with the change.

Detailed description of the issue can be found here

/cc @prajnamas @orozot @qk44077907 @lxwan

Loop @ntepluhina @leipert since the MR was created/reviewed by you.

Commit messages

Fix: gettext:po_to_json deprecation issue

• Remove the old gettext:po_to_json due to potential deprecation.
• Use newly introduced task to compile locale files.
• Update documentation for the changes.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Pre-request

Turn off assets caching settings in config/gdk.yml

1. Check out to the master branch then change the preferred locale to any language other than English.
2. Go to any issue/MR page to see if the content was mixed with both the preferred language and English.
3. Check out to this branch and then reboot the rails-web service with your GDK.
4. Refresh the page to see if the issue remains.

MR acceptance checklist

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