tails-upgrade-frontend-wrapper points users to a 404 URL for manual upgrades in French

Hi,

Tails-upgrade-frontend-wrapper redirects users to a 404 URL for manual upgrades, for FR_fr, because of,

• An outdated URL in the translation (po/fr.po): https://tails.boum.org/doc/first_steps/upgrade/index.fr.html/#manual
• A redirect rules that does not take account language specific index.XX.html pages (zh_CN.po contains https://tails.boum.org/doc/first_steps/upgrade/ which is properly redirected).

It may worth to,

• update the URL
• update the redirect rules to take account language specific pages, depending how old is the renaming (I was not able to find it, git log is enormous for this page !)

@sajolida @emmapeel maybe will you be interested to give a look.

Originally reported on twitter https://twitter.com/vincib/status/1311672899280015361.