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/openvpn/start.go

@@ -2,15 +2,12 @@ package openvpn
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os/exec"
 
 	"github.com/qdm12/gluetun/internal/constants/openvpn"
 )
 
-var ErrVersionUnknown = errors.New("OpenVPN version is unknown")
-
 const (
 	binOpenvpn25 = "openvpn2.5"
 	binOpenvpn26 = "openvpn2.6"
@@ -26,7 +23,7 @@ func start(ctx context.Context, starter CmdStarter, version string, flags []stri
 	case openvpn.Openvpn26:
 		bin = binOpenvpn26
 	default:
-		return nil, nil, nil, fmt.Errorf("%w: %s", ErrVersionUnknown, version)
+		return nil, nil, nil, fmt.Errorf("OpenVPN version is unknown: %s", version)
 	}
 
 	args := []string{"--config", configPath}