Password length can be set to a value too low to ever work

[brianhogg]

Reproduction Steps

1. Under LifterLMS > Forms, edit the registration form
2. Set the minimum password length to a low value (ie. 1)
3. Attempt to have a one-letter password

Expected Behavior

User is able to enter a 1-character password

Actual Behavior

"Too short" message displays

Tech Notes

It looks like 6 is the absolute minimum length. We should make the "min" value "6".