Update CNS validation logic

[geidsonc]

Updated the isValidCns() function in src/Helpers/Validate.php to include CNS numbers starting with 5 in the temporary validation logic. Also updated tests/Helpers/ValidateTest.php with valid and invalid examples for CNS starting with 5 as requested.

Fixes #59

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PR created automatically by Jules for task 342378906075137649 started by @geidsonc

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[geidsonc]

@jules update this branch with master