Remove usage of the `doc_auto_cfg` feature #5786

Job	Run time
cargo-hack	2m 36s
check	42s
check-docs	28s
cargo-public-api-crates (axum-macros)	35s
cargo-public-api-crates (axum)	51s
cargo-public-api-crates (axum-core)	35s
cargo-public-api-crates (axum-extra)	49s
cargo-deny check (bans licenses sources)	23s
cargo-deny check (advisories)	27s
dependencies-are-sorted	7s
Spell Check with Typos	6s
test-nightly	1m 1s
test-versions (beta)	1m 14s
test-docs	59s
test-versions (stable)	1m 12s
armv5te-unknown-linux-musleabi	35s
test-msrv	3m 4s
wasm32-unknown-unknown	19s
	16m 3s

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