switch lint config to `manifest-lint` feature

I removed some lint configuration in the process:

* `#[allow(clippy::suspicious_else_formatting)]` because nothing is
  currently triggering it.
* `#[warn(clippy::future_not_send)]` because some stuff under
  `src/lib.rs` is. And also like, auto-trait leakage generally means
  this isn't a problem, and if things really need to be `Send`, then
  you'll probably know to mark it manually.
* `#[warn(rust_2018_idioms)]` and replaced it with
  `explicit-outlives-requirements = "warn"` which is the most useful
  lint in that group that isn't enabled by default.

Cargo.toml

@@ -1,3 +1,14 @@
+# Keep alphabetically sorted
+[workspace.lints.rust]
+explicit_outlives_requirements = "warn"
+unused_qualifications = "warn"
+
+# Keep alphabetically sorted
+[workspace.lints.clippy]
+cloned_instead_of_copied = "warn"
+dbg_macro = "warn"
+str_to_string = "warn"
+
 [package]
 name = "conduit"
 description = "A Matrix homeserver written in Rust"
@@ -14,6 +25,9 @@ rust-version = "1.75.0"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[lints]
+workspace = true
+
 [dependencies]
 # Web framework
 axum = { version = "0.6.18", default-features = false, features = ["form", "headers", "http1", "http2", "json", "matched-path"], optional = true }

src/lib.rs

@@ -1,12 +1,3 @@
-#![warn(
-    rust_2018_idioms,
-    unused_qualifications,
-    clippy::cloned_instead_of_copied,
-    clippy::str_to_string
-)]
-#![allow(clippy::suspicious_else_formatting)]
-#![deny(clippy::dbg_macro)]
-
 pub mod api;
 mod config;
 mod database;

src/main.rs

@@ -1,13 +1,3 @@
-#![warn(
-    rust_2018_idioms,
-    unused_qualifications,
-    clippy::cloned_instead_of_copied,
-    clippy::str_to_string,
-    clippy::future_not_send
-)]
-#![allow(clippy::suspicious_else_formatting)]
-#![deny(clippy::dbg_macro)]
-
 use std::{future::Future, io, net::SocketAddr, sync::atomic, time::Duration};
 
 use axum::{