fix(serde-querystring-axum): update dependencies

[jackssrt]

this pr:

• updates axum-core
• updates http
• removes async-trait (no longer needed since async fn in traits got stabilized a while ago)
• adds http-body-util as a dev dependency for testing the outgoing request bodies
• simplifies the hidden sections of the doctests that used axum::Server

[pooyamb]

@jackssrt Thanks for your contribution. I will release a new version in the next week or two, hopefully.