Remove unused rustversion dependency of axum-core #5780

Job	Run time
check	41s
check-docs	29s
cargo-hack	2m 57s
cargo-public-api-crates (axum-core)	17s
cargo-public-api-crates (axum-macros)	19s
cargo-public-api-crates (axum)	36s
cargo-public-api-crates (axum-extra)	27s
dependencies-are-sorted	7s
cargo-deny check (advisories)	24s
Spell Check with Typos	8s
cargo-deny check (bans licenses sources)	30s
test-nightly	1m 9s
test-versions (beta)	1m 9s
test-versions (stable)	1m 11s
armv5te-unknown-linux-musleabi	34s
test-docs	58s
wasm32-unknown-unknown	25s
test-msrv	2m 5s
	14m 26s

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