Drop Nim 1.6 support #300

Job	Run time
build / linux-gcc-14-amd64 (Nim devel)	4m 34s
build / linux-amd64 (Nim version-2-0)	4m 25s
build / windows-amd64 (Nim devel)	4m 32s
build / linux-gcc-14-amd64 (Nim version-2-2)	4m 35s
build / linux-i386 (Nim version-2-2)	4m 36s
build / linux-gcc-14-amd64 (Nim version-2-0)	4m 36s
build / macos-arm64 (Nim version-2-2)	4m 45s
build / macos-amd64 (Nim version-2-0)	4m 37s
build / windows-amd64 (Nim version-2-2)	4m 38s
build / macos-amd64 (Nim devel)	4m 39s
build / macos-arm64 (Nim devel)	4m 44s
build / linux-amd64 (Nim devel)	4m 38s
build / linux-i386 (Nim version-2-0)	4m 36s
build / windows-amd64 (Nim version-2-0)	4m 37s
build / linux-i386 (Nim devel)	4m 35s
build / macos-arm64 (Nim version-2-0)	4m 44s
build / linux-amd64 (Nim version-2-2)	4m 35s
build / macos-amd64 (Nim version-2-2)	4m 39s
	1h 23m 5s

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