Fix intra-doc links on docs.rs (#1205)

* Use version 2 of Cargo's feature resolver

* Increase minimum version of tungstenite

Tungstenite 0.17.1 has a higher MSRV, and there should be no reason to
use it over 0.17.2.

* Clean up and fix MSRV CI job

* Fix some intra-doc links not resolving correctly on docs.rs

* Bump minimum version of tower

… to avoid dependencies with a broken minimum-versions chain.

Cargo.toml

@@ -4,9 +4,5 @@ members = [
     "axum-core",
     "axum-extra",
     "axum-macros",
-
-    # internal crate used to bump the minimum versions we
-    # get for some dependencies which otherwise wouldn't build
-    # with `cargo +nightly update -Z minimal-versions`
-    "internal-minimal-versions",
 ]
+resolver = "2"