Update tower version to fix build issue

[ShawnROGrady]

Bug Report

Version

tonic v0.4.0

Description

I tried updating tonic for one of my projects to finish migrating to v1 of tokio and began seeing build issues:

Compiling tower v0.4.0
error[E0432]: unresolved imports `tower_layer::layer_fn`, `tower_layer::LayerFn`
 --> /Users/Shawn/.cargo/registry/src/jackfan.us.kg-1ecc6299db9ec823/tower-0.4.0/src/util/boxed/sync.rs:1:19
  |
1 | use tower_layer::{layer_fn, LayerFn};
  |                   ^^^^^^^^  ^^^^^^^
  |                   |         |
  |                   |         no `LayerFn` in the root
  |                   |         help: a similar name exists in the module: `Layer`
  |                   no `layer_fn` in the root

error[E0432]: unresolved imports `tower_layer::layer_fn`, `tower_layer::LayerFn`
 --> /Users/Shawn/.cargo/registry/src/jackfan.us.kg-1ecc6299db9ec823/tower-0.4.0/src/util/boxed/unsync.rs:1:19
  |
1 | use tower_layer::{layer_fn, LayerFn};
  |                   ^^^^^^^^  ^^^^^^^
  |                   |         |
  |                   |         no `LayerFn` in the root
  |                   |         help: a similar name exists in the module: `Layer`
  |                   no `layer_fn` in the root

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0432`.
error: could not compile `tower`.

It appears as though this issue has been fixed in a newer version of tower (release: https://github.com/tower-rs/tower/releases/tag/tower-0.4.1, commit: tower-rs/tower@b447771)

EDIT: this may be a cargo issue because explicitly adding tower = { version = "0.4.1" } to my Cargo.toml (I'm not directly depending on tower so I didn't previously have this dependency) resolved the issue. Still might be best to update tonic/Cargo.toml to set the tower version to ^0.4.1 so others don't run in to similar issues updating.