Write migration doc #2271

Job	Run time
Docs	17s
build_versions (stable)	19s
build_versions (beta)	19s
Test	38s
cargo-deny (aarch64-apple-ios)	19s
build_versions (1.67.0)	26s
Lint	19s
Test (charset)	36s
cargo-deny (aarch64-linux-android)	18s
Test (cookies)	40s
cargo-deny (i686-pc-windows-gnu)	19s
Test (socks-proxy)	39s
cargo-deny (i686-pc-windows-msvc)	16s
Test (gzip)	37s
cargo-deny (i686-unknown-linux-gnu)	18s
Test (brotli)	37s
cargo-deny (wasm32-unknown-unknown)	15s
Test (json)	36s
cargo-deny (x86_64-apple-darwin)	21s
Test (native-tls)	40s
cargo-deny (x86_64-apple-ios)	19s
cargo-deny (x86_64-pc-windows-gnu)	19s
cargo-deny (x86_64-pc-windows-msvc)	17s
cargo-deny (x86_64-unknown-linux-gnu)	19s
cargo-deny (x86_64-unknown-redox)	16s
	10m 19s

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