Bump base bound to 4.21 for GHC 9.12 #51

Job	Run time
emulated (s390x)	35s
arm (arm32v7)	10m 21s
darwin_arm (self-hosted, macOS, ARM64, 8.10.7)	6h 0m 14s
emulated (ppc64le)	32s
arm (arm64v8)	9m 2s
build (ubuntu-latest, 8.6, 3.8.1.0)	13m 34s
darwin_arm (self-hosted, macOS, ARM64, 9.2.8)	21m 55s
build (ubuntu-latest, 8.8, 3.8.1.0)	10m 38s
darwin_arm (self-hosted, macOS, ARM64, 9.4.7)	23m 19s
build (ubuntu-latest, 8.10, 3.8.1.0)	10m 21s
i386	15m 3s
freebsd (self-hosted, FreeBSD, X64, 9.4.8)	6m 11s
build (ubuntu-latest, 9.0, 3.8.1.0)	10m 20s
freebsd (self-hosted, FreeBSD, X64, 9.6.4)	3h 53m 13s
build (ubuntu-latest, 9.2, 3.8.1.0)	9m 0s
build (ubuntu-latest, 9.4, 3.8.1.0)	11m 57s
build (ubuntu-latest, 9.6, 3.8.1.0)	14m 8s
build (ubuntu-latest, 9.8, 3.8.1.0)	8m 26s
build (macOS-12, 9.4, 3.8.1.0)	37m 11s
build (macOS-12, 9.6, 3.8.1.0)	47m 38s
build (windows-latest, 9.4, 3.8.1.0)	51m 22s
build (windows-latest, 9.6, 3.8.1.0)	48m 2s
	15h 53m 2s

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