Disallow nesting at the root of the router #5060

Job	Run time
cargo-hack	5m 52s
check-docs	29s
cargo-public-api-crates (axum)	35s
check	1m 50s
cargo-public-api-crates (axum-core)	18s
cargo-deny check (advisories)	28s
dependencies-are-sorted	12s
cargo-public-api-crates (axum-extra)	29s
cargo-deny check (bans licenses sources)	26s
Spell Check with Typos	4s
cargo-public-api-crates (axum-macros)	19s
test-msrv	1m 46s
test-versions (stable)	3m 32s
test-versions (beta)	2m 35s
test-docs	1m 40s
armv5te-unknown-linux-musleabi	46s
test-nightly	1m 37s
wasm32-unknown-unknown	25s
	23m 23s

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