Updated related_to_banned_user? method in Users::PhoneNumberValidation

What does this MR do and why?

• Updates the related_to_banned_user? method in Users::PhoneNumberValidation
• It only returns true if the matching phone number record has been verified, i.e. validated_at column of the matching record is not nil
• This is in response to an incident. We noticed that malicious users were entering random phone numbers, then getting blocked, which was in-turn preventing legitimate users from signing up.
• Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/530

Database

• Current query plan
• Proposed query plan

How to set up and validate locally

1. In the rails console:

# create a phone number validation record
> Users::PhoneNumberValidation.create(international_dial_code: '1', phone_number: '333', country: 'US', user: User.last, validated_at: nil)

# ban the user
User.last.ban!

# check if the phone number matches a banned user, should return false
> Users::PhoneNumberValidation.related_to_banned_user?('1', '333')
=> false 

# Update validated_at of the record
Users::PhoneNumberValidation.last.update(validated_at: Time.now)

# try again, should return true
> Users::PhoneNumberValidation.related_to_banned_user?('1', '333')
=> true 

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.