This PR updates the regex pattern used for validating French postal codes in the isPostalCode function.

• Replaces outdated regex (/^\d{2}\s?\d{3}$/) with accurate pattern (/^(?:(?:0[1-9]|[1-8]\d|9[0-5])\d{3}|97[1-46]\d{2})$/)
• Updates the test fixtures in validators.test.js to ensure accurate testing of French postal code validation.

Fixes #2478

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)