Add password rule for vanguard.com

[ulysse-oriol]

Overall Checklist

• I agree to the project's Developer Certificate of Origin
• The top-level JSON objects are sorted alphabetically
• There are no open pull requests for the same update

for password-rules.json

• The given rule isn't particularly standard and obvious for password managers
• Generated passwords have been tested from this rule using the Password Rules Validation Tool
• Information has been included about the website's requirements (eg. screenshots, error messages, steps during experimentation, etc.)
• The PR isn't documenting something that would be a common practice among password managers (e.g. minimal length of 6)

Addresses issue: #954

https://security-settings.web.vanguard.com/username-password

The rules given on the website:

I wasn't able to test it on the website itself since it redirects to https://login.vanguard.com.