Update which requirement from ^4.0 to ^6.0 #1026

Job	Run time
Test CUDA support	34s
Clippy	9s
Test aarch64-apple-tvos	2m 10s
MSRV (ubuntu-latest)	46s
Rustfmt	13s
MSRV (windows-latest)	1m 30s
Test (stable)	29s
Test (beta)	39s
Test (nightly)	37s
Test (linux32)	49s
Test (macos)	1m 9s
Test (aarch64-ios)	1m 20s
Test (win32)	1m 40s
Test (win64)	2m 21s
Test (mingw32)	1m 59s
Test (mingw64)	1m 39s
Test (windows-2019)	2m 25s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 15s
	22m 44s

Provide feedback