go: lower Go requirement to Go 1.18 #236

Job	Run time
codespell	6s
test-windows (go.mod)	10s
test-unix (ubuntu-latest, go.mod)	4s
test-windows (1.20)	57s
test-build (windows-latest, 1.18)	37s
test-unix (ubuntu-latest, 1.20)	10m 6s
test-windows (1.21)	1m 16s
test-build (windows-latest, 1.19)	32s
test-unix (ubuntu-latest, 1.21)	10m 6s
test-windows (oldstable)	35s
test-build (windows-latest, 1.20)	1m 7s
test-unix (ubuntu-latest, oldstable)	9m 50s
test-windows (stable)	1m 35s
test-build (windows-latest, 1.21)	29s
test-unix (ubuntu-latest, stable)	9m 50s
test-build (windows-latest, 1.22)	1m 21s
test-unix (macos-latest, go.mod)	6s
test-unix (macos-latest, 1.20)	24s
test-build (windows-latest, 1.23)	31s
test-unix (macos-latest, 1.21)	9s
test-build (windows-latest, ^1)	1m 7s
test-unix (macos-latest, oldstable)	11s
test-build (ubuntu-latest, 1.18)	17s
test-unix (macos-latest, stable)	10s
test-build (ubuntu-latest, 1.19)	18s
test-build (ubuntu-latest, 1.20)	12s
test-build (ubuntu-latest, 1.21)	10s
test-build (ubuntu-latest, 1.22)	10s
test-build (ubuntu-latest, 1.23)	11s
test-build (ubuntu-latest, ^1)	4s
test-build (macos-latest, 1.18)	24s
test-build (macos-latest, 1.19)	18s
test-build (macos-latest, 1.20)	16s
test-build (macos-latest, 1.21)	8s
test-build (macos-latest, 1.22)	9s
test-build (macos-latest, 1.23)	11s
test-build (macos-latest, ^1)	8s
coverage	0s
	54m 15s

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