chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/firewall/iptables/tcp.go

@@ -45,12 +45,10 @@ func (f tcpFlag) String() string {
 	case tcpFlagCWR:
 		return "CWR"
 	default:
-		panic(fmt.Sprintf("%s: %d", errTCPFlagUnknown, f))
+		panic(fmt.Sprintf("unknown TCP flag: %d", f))
 	}
 }
 
-var errTCPFlagUnknown = errors.New("unknown TCP flag")
-
 func parseTCPFlag(s string) (tcpFlag, error) {
 	allFlags := []tcpFlag{
 		tcpFlagFIN, tcpFlagSYN, tcpFlagRST, tcpFlagPSH,
@@ -61,7 +59,7 @@ func parseTCPFlag(s string) (tcpFlag, error) {
 			return flag, nil
 		}
 	}
-	return 0, fmt.Errorf("%w: %s", errTCPFlagUnknown, s)
+	return 0, fmt.Errorf("unknown TCP flag: %s", s)
 }
 
 var ErrMarkMatchModuleMissing = errors.New("kernel is missing the mark module libxt_mark.so")