Migrate sentinels to use Python 3.15's PEP 661 `sentinel` built-in with typing_extensions backport

Migrate sentinel values like at `src/click/_utils.py` to use the introduced `sentinel` built-in coming in Python 3.15. For older versions we can use [`typing_extenions.sentinel`](https://typing-extensions.readthedocs.io/en/latest/index.html#typing_extensions.sentinel).