Move Request.headers back to Mapping

[nateprewitt]

This PR partially reverts #7431, moving it back to Mapping instead of MutableMapping. While we typically expect the input to be mutable, dicts inferred to be dict[str, str] at creation are incompatible with MutableMapping[str, str | bytes] even though they may later have a bytes value added.

This typing decision is a tradeoff of supporting update() calls on Request.headers and requiring all users to type their input as dict[str, str | bytes]. The latter is significantly more invasive and common for current codebases.