Update which requirement from ^4.0 to ^6.0 #1051

Job	Run time
Test CUDA support	23s
Test (stable)	28s
Test aarch64-apple-tvos	2m 58s
Test (beta)	35s
Clippy	14s
Test (nightly)	35s
MSRV (ubuntu-latest)	1m 50s
Rustfmt	9s
MSRV (windows-latest)	2m 10s
Test (linux32)	48s
Test (macos)	1m 10s
Test (aarch64-ios)	3m 41s
Test (win32)	1m 31s
Test (win64)	2m 15s
Test (mingw32)	1m 30s
Test (mingw64)	1m 37s
Test (windows-2019)	1m 17s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 6s
	25m 17s

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