locale_gen: works with C.UTF-8 (#6774)
* locale_gen: fix
* test working with C.UTF-8
* working with locale eo
* handle C.UTF-8 edge cases
* grammatic pedantism
* add changelog frag
* add doc about specific OS support
* update changelog frag
(cherry picked from commit 3fd4cdb119)
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
