Update example reverse-proxy to axum 0.7 #4128

Job	Run time
check-docs	24s
check	1m 4s
cargo-public-api-crates (axum)	39s
cargo-hack	3m 16s
cargo-public-api-crates (axum-core)	25s
cargo-deny check (advisories)	56s
cargo-public-api-crates (axum-extra)	35s
cargo-deny check (bans licenses sources)	28s
dependencies-are-sorted	27s
cargo-public-api-crates (axum-macros)	30s
Spell Check with Typos	7s
test-msrv	1m 56s
test-nightly	1m 59s
test-docs	2m 27s
armv5te-unknown-linux-musleabi	57s
test-versions (stable)	3m 26s
wasm32-unknown-unknown	23s
test-versions (beta)	2m 54s
	22m 53s

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