Update PHP Version and Refactoring

• Updating the minimum version of PHP to 7.4
• Validate the signing algorithm against the supported algorithms https://github.com/see https://www.rfc-editor.org/rfc/rfc7518#section-3
• Sanitize the REQUEST_URI and HTTP_AUTHORIZATION values before using them
• Use get_header() instead of $_SERVER to get the Authorization header when possible
• Added typed properties to the JWT_Auth class where possible
• Along with this release, I release a new simple JWT Client App for testing purposes https://github.com/see https://github.com/Tmeister/jwt-client
• WP Code Standards
• Remove unnecessary tests