Move axum-extra's Html response into axum

[jplatte]

In #1921, David added an Html response type that adds ; charset=utf-8 to the content-type (this might need further discussion) and has a different bound on the inner type than axum's Html response type. It was meant to be replace the latter for axum 0.7.0 originally, but was forgotten for that release.

We should move it with the next breaking-change release.