refactor: replace trait fn bounds with impl AsyncFn

[alwayys-afk]

Some of the tests had this pattern:

async fn request_internal<F, Fut>(callback: F, boot: &BootResult, test_server_config: RequestConfig)
where
    F: FnOnce(TestServer, AppContext) -> Fut,
    Fut: std::future::Future<Output = ()>,

switched to this:

async fn request_internal(
    callback: impl AsyncFnOnce(TestServer, AppContext),
    boot: &BootResult,
    test_server_config: RequestConfig)

[dsgallups]

looks good, though the PR description doesn't quite match what this does.
Before:

request::<App, _, _>(|test_server, etc)| async { ... });

This PR:

request::<App>(|test_server, etc)| async { ... });