go: lower Go requirement to Go 1.18 #237

Job	Run time
codespell	3s
test-windows (1.18)	29s
test-unix (ubuntu-latest, 1.18)	13s
test-windows (1.20)	1m 18s
test-build (windows-latest, 1.18)	32s
test-unix (ubuntu-latest, 1.20)	10m 16s
test-windows (1.21)	37s
test-build (windows-latest, 1.19)	38s
test-unix (ubuntu-latest, 1.21)	10m 21s
test-windows (oldstable)	1m 7s
test-build (windows-latest, 1.20)	1m 4s
test-unix (ubuntu-latest, oldstable)	10m 6s
test-windows (stable)	34s
test-build (windows-latest, 1.21)	1m 14s
test-unix (ubuntu-latest, stable)	10m 15s
test-build (windows-latest, 1.22)	1m 37s
test-unix (macos-latest, 1.18)	24s
test-build (windows-latest, 1.23)	33s
test-unix (macos-latest, 1.20)	19s
test-build (windows-latest, ^1)	57s
test-unix (macos-latest, 1.21)	12s
test-build (ubuntu-latest, 1.18)	13s
test-unix (macos-latest, oldstable)	11s
test-build (ubuntu-latest, 1.19)	14s
test-unix (macos-latest, stable)	13s
test-build (ubuntu-latest, 1.20)	11s
test-build (ubuntu-latest, 1.21)	9s
test-build (ubuntu-latest, 1.22)	6s
test-build (ubuntu-latest, 1.23)	9s
test-build (ubuntu-latest, ^1)	4s
test-build (macos-latest, 1.18)	31s
test-build (macos-latest, 1.19)	27s
test-build (macos-latest, 1.20)	19s
test-build (macos-latest, 1.21)	11s
test-build (macos-latest, 1.22)	9s
test-build (macos-latest, 1.23)	9s
test-build (macos-latest, ^1)	9s
coverage	0s
	56m 14s

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