Use POST instead GET in check entropy and HIBP modules

The entropy indicator was introduced in 2.18 (see #2971 (closed))

The checks are done by sending the password with GET, resulting in having the password in access logs:

auth.example.com:80 127.0.0.1 - - [10/Apr/2024:10:53:12 +0200] "GET /checkentropy?password=dGVzdA== HTTP/1.1" 200 658 "http://auth.example.com/?cancel=1&skin=bootstrap" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:123.0) Gecko/20100101 Firefox/123.0"

It would be better to use POST