Fix the password length checks on invite User

[peterhashair]

Description:

Fixes: #19573
update password checks

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[bx80]

It would be great if we could have the acceptInvitation method call UsersManager::checkPassword instead of duplicating the same checks, as suggested in the issue description. There might be additional password requirements in the future, so having just one place that decides if a password is valid would be more future proof. checkPassword throws an exception instead of providing a return value, but acceptInvitation could catch the exception string and set it as the value of $error.