Upgrade Rust toolchain to `nightly-2024-03-21` #6394

Job	Run time
benchcomp-tests	3m 27s
bookrunner	17m 5s
perf	29m 10s
regression (macos-13)	27m 1s
write-json-symtab-regression	13m 28s
regression (ubuntu-20.04)	21m 49s
regression (ubuntu-22.04)	23m 51s
	2h 15m 51s

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