Remove transitive dependency on mio

This allows projects targeting `wasm32-unknown-unknown` to compile.

The `mio` dependency is being brought in via: `cornucopia_async`→`tokio-postgres`→`tokio-postgres/runtime`→`tokio/net`→`mio`. By disabling the default features of `tokio-postgres`, we disable its `runtime` feature.

crates/client_async/Cargo.toml

@@ -25,7 +25,7 @@ cornucopia_client_core = { path = "../client_core", version = "0.4.0" }
 async-trait = "0.1.63"
 
 # rust-postgres interaction
-tokio-postgres = "0.7.7"
+tokio-postgres = { version = "0.7.7", default-features = false }
 
 # connection pooling
 deadpool-postgres = { version = "0.12.1", optional = true }