Fix `opaque type` warnings about torrust-tracker

[josecelano]

We got rid of the first warning:

warning: opaque type `impl warp::Filter + Clone + warp::filter::FilterBase<Extract = impl Reply, Error = Rejection>` does not satisfy its associated type bounds
  --> src/api/routes.rs:50:63
   |
50 | pub fn routes(tracker: &Arc<tracker::Tracker>) -> impl Filter<Extract = impl warp::Reply, Error = warp::Rejection> + Clone {
   |                                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
  ::: /home/josecelano/.cargo/registry/src/github.com-1ecc6299db9ec823/warp-0.3.3/src/filter/mod.rs:40:19
   |
40 |     type Extract: Tuple; // + Send;
   |                   ----- this associated type bound is unsatisfied for `impl Reply`
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
help: add this bound
   |
50 | pub fn routes(tracker: &Arc<tracker::Tracker>) -> impl Filter<Extract = impl warp::Reply + warp::generic::Tuple, Error = warp::Rejection> + Clone {
   |   

because we have merged the PR replacing the Warp API implementation with the new Axum implementation.

[josecelano]

This will be fixed with #160