chacha20poly1305: update URL for `draft-irtf-cfrg-xchacha` (#608) #899

Job	Run time
build (1.65.0, armv7a-none-eabi)	2m 1s
test (i686-unknown-linux-gnu, 1.72.0, sudo apt update && sudo apt install gcc-multilib)	45s
build (1.65.0, thumbv7em-none-eabi)	1m 57s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 2s
build (1.65.0, wasm32-unknown-unknown)	1m 57s
test (x86_64-unknown-linux-gnu, 1.72.0)	29s
build (stable, armv7a-none-eabi)	10s
test (x86_64-unknown-linux-gnu, stable)	22s
build (stable, thumbv7em-none-eabi)	52s
build (stable, wasm32-unknown-unknown)	9s
	9m 44s

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