Add `must_use` attribute to `Serve` #4285

Job	Run time
cargo-hack	2m 51s
check	44s
check-docs	25s
cargo-public-api-crates (axum)	33s
cargo-public-api-crates (axum-core)	18s
cargo-deny check (advisories)	23s
dependencies-are-sorted	19s
cargo-public-api-crates (axum-extra)	31s
Spell Check with Typos	4s
cargo-deny check (bans licenses sources)	23s
cargo-public-api-crates (axum-macros)	17s
test-nightly	2m 4s
test-msrv	2m 10s
test-docs	1m 30s
armv5te-unknown-linux-musleabi	55s
test-versions (stable)	2m 2s
test-versions (beta)	3m 11s
wasm32-unknown-unknown	23s
	19m 3s

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