Bump `io-classes` and co to `1.6` #287

Job	Run time
cabal-check (9.6.4, 3.10.3.0, ubuntu-latest)	2m 41s
stylish-haskell (9.6.4, 3.10.3.0, ubuntu-latest)	2m 41s
build (8.10.7, 3.10.3.0, ubuntu-latest)	1m 57s
cabal-fmt (9.6.4, 3.10.3.0, ubuntu-latest)	2m 49s
build (8.10.7, 3.10.3.0, windows-latest)	4m 36s
build (8.10.7, 3.10.3.0, macOS-13)	4m 8s
build (9.2.8, 3.10.3.0, ubuntu-latest)	2m 3s
build (9.2.8, 3.10.3.0, windows-latest)	4m 52s
build (9.2.8, 3.10.3.0, macOS-13)	2m 44s
build (9.4.8, 3.10.3.0, ubuntu-latest)	2m 42s
build (9.4.8, 3.10.3.0, windows-latest)	4m 7s
build (9.4.8, 3.10.3.0, macOS-13)	6m 33s
build (9.6.4, 3.10.3.0, ubuntu-latest)	3m 0s
build (9.6.4, 3.10.3.0, windows-latest)	4m 31s
build (9.6.4, 3.10.3.0, macOS-13)	5m 56s
build (9.8.2, 3.10.3.0, ubuntu-latest)	1m 40s
build (9.8.2, 3.10.3.0, windows-latest)	5m 44s
build (9.8.2, 3.10.3.0, macOS-13)	6m 34s
	1h 9m 18s

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