misc: mark Body::channel() as deprecated in 0.14

[cratelyn]

this commit marks Body::channel() as deprecated in the 0.14 release.

#2970 (see #2962) made this interface private, and hyper 1.0 no longer includes this function. instead, users should wrap an mpsc::Receiver<T> in a StreamBody. this should help users migrating from hyper 0.14 to 1.0, by indicating that this interface is affected by the breaking changes in hyper 1.0, and indicating a suitable upgrade path.

[cratelyn]

with hyperium/http-body#140 landed, there's now a much cleaner path for migration. calls to hyper::Body::channel() can be replaced with http_body_util::channel::Channel::new(..). this mostly aligns with other types that have moved out to libraries like hyper_util or http_body_util.

now that there is a more direct 1:1 equivalent for those migrating to use, i think this can be closed.