Fix user creation with stronger minimum password requirements

What does this MR do?

doc/security/password_length_limits.md Describes a way to increase minimum requirements of passwords, via config/initializers/devise_password_length.rb. However, user creation hardcoded password generation at 8 characters.

If the minimum was set at any more than 8 characters, user creation will fail.

Function now looks up minimum length requirement.

What are the relevant issue numbers?

Fixes #14563 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together