ci: supports building with Go 1.20 and raises floor version to 1.18 (#1096)

This moves our floor version to the same we'll release 1.0 with: 1.18.
This is congruent with our version policy which is current-2.

Fixes #921

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/gojs/time_test.go

@@ -26,8 +26,8 @@ func Test_time(t *testing.T) {
 1ms
 `, stdout)
 
-	// Search for the three functions that should be in scope, flexibly, to pass
-	// go 1.17-19
+	// To avoid multiple similar assertions, just check three functions we
+	// expect were called.
 	require.Contains(t, log.String(), `==> go.runtime.nanotime1()
 <== (nsec=0)`)
 	require.Contains(t, log.String(), `==> go.runtime.walltime()