refactor: clean up unused code, fix typos, improve docs

- Remove unused 'killed' const and 'unitTypes' var (staticcheck U1000)
- Replace regexp with strings.TrimSuffix+switch in isFailed for consistency
- Fix typo: 'programatically' -> 'programmatically'
- Fix typo: 'an an int' -> 'as an int' in README and helpers.go
- Add missing godoc comments on exported helper functions
- Bump minimum Go version from 1.18 to 1.21

util.go

@@ -11,8 +11,6 @@ import (
 
 var systemctl string
 
-const killed = 130
-
 func init() {
 	path, _ := exec.LookPath("systemctl")
 	systemctl = path