Unwrap .spawn() to support updated tokio task builder API

[zackradisic]

When compiling serenity with the tokio_task_builder feature enabled, the following error occurs:

error[E0308]: mismatched types
 --> /Users/zackradisic/.cargo/registry/src/github.com-1ecc6299db9ec823/serenity-0.11.5/src/internal/tokio.rs:9:5
  |
4 | pub fn spawn_named<F, T>(name: &str, future: F) -> tokio::task::JoinHandle<T>
  |                                                    -------------------------- expected `tokio::task::JoinHandle<T>` because of return type
...
9 |     tokio::task::Builder::new().name(&*format!("serenity::{}", name)).spawn(future)
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `tokio::task::JoinHandle`, found enum `std::result::Result`
  |
  = note: expected struct `tokio::task::JoinHandle<_>`
               found enum `std::result::Result<tokio::task::JoinHandle<_>, std::io::Error>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `serenity` due to previous error

In this commit tokio's task::Builder::spawn* methods were changed to return an std::io::Result<_> to allowing opt-in to gracefully handling errors when spawning tasks (PR). According to git it has been present since tokio version 1.21.0

The PR says the previous behaviour was to simply panic on failure, so a simple fix is to do the same and .unwrap() the value returned from spawning

[mkrasnitski]

This is alright to do on current (since the build is otherwise broken), but on next it might be better to change the function signature to return a Result<JoinHandle<T>>.