Turn default hash password algorightm back to pbkdf2 from argon2 until we found a better one (#14673)

* Turn default hash password algorightm back to pbkdf2 from argon2 until we found a better one

* Add a warning on document

custom/conf/app.example.ini

@@ -565,7 +565,7 @@ ONLY_ALLOW_PUSH_IF_GITEA_ENVIRONMENT_SET = true
 ;Classes include "lower,upper,digit,spec"
 PASSWORD_COMPLEXITY = off
 ; Password Hash algorithm, either "argon2", "pbkdf2", "scrypt" or "bcrypt"
-PASSWORD_HASH_ALGO = argon2
+PASSWORD_HASH_ALGO = pbkdf2
 ; Set false to allow JavaScript to read CSRF cookie
 CSRF_COOKIE_HTTP_ONLY = true
 ; Validate against https://haveibeenpwned.com/Passwords to see if a password has been exposed