Refactor signedCookies function for improved readability

[Ayoub-Mabrouk]

• Replaced Object.keys() with Object.entries() for cleaner iteration over key-value pairs.
• Used destructuring in the loop to enhance code clarity.
• Maintained functionality of decoding signed cookies while removing them from the original object.

[UlisesGascon]

Same as https://github.com/expressjs/cookie-parser/pull/119#issuecomment-2451729449