Update isValidCns logic to include CNS starting with 5 #59

I need to update isValidCns() function on src/Helpers/Validate.php

Currently, the function validate_cns_tmp() only triggers for CNS numbers starting in 7, 8, or 9. Based on this documentation, numbers starting with 5 are now also valid and should follow the same validation logic.

I want you to locate where the conditional responsible for checking this out and add 5 to allowed digits.

To assure implementation is correct, I want you to generate/update test suites with the following examples:
Valid ones:

• 550354737380003
• 521473152170018
  Invalid ones:
• 503521996570006
• 556492895730003
• 584480646290007