chore(config): use openvpn protocol string field instead of TCP bool

internal/configuration/sources/env/openvpnselection.go

@@ -1,12 +1,7 @@
 package env
 
 import (
-	"errors"
-	"fmt"
-	"strings"
-
 	"github.com/qdm12/gluetun/internal/configuration/settings"
-	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gosettings/sources/env"
 )
 
@@ -14,7 +9,7 @@ func (s *Source) readOpenVPNSelection() (
 	selection settings.OpenVPNSelection, err error) {
 	selection.ConfFile = s.env.Get("OPENVPN_CUSTOM_CONFIG", env.ForceLowercase(false))
 
-	selection.TCP, err = s.readOpenVPNProtocol()
+	selection.Protocol = s.env.String("OPENVPN_PROTOCOL", env.RetroKeys("PROTOCOL"))
 	if err != nil {
 		return selection, err
 	}
@@ -29,28 +24,3 @@ func (s *Source) readOpenVPNSelection() (
 
 	return selection, nil
 }
-
-var ErrOpenVPNProtocolNotValid = errors.New("OpenVPN protocol is not valid")
-
-func (s *Source) readOpenVPNProtocol() (tcp *bool, err error) {
-	const currentKey = "OPENVPN_PROTOCOL"
-	envKey := firstKeySet(s.env, "PROTOCOL", currentKey)
-	switch envKey {
-	case "":
-		return nil, nil //nolint:nilnil
-	case currentKey:
-	default: // Retro compatibility
-		s.handleDeprecatedKey(envKey, currentKey)
-	}
-
-	protocol := s.env.String(envKey)
-	switch strings.ToLower(protocol) {
-	case constants.UDP:
-		return ptrTo(false), nil
-	case constants.TCP:
-		return ptrTo(true), nil
-	default:
-		return nil, fmt.Errorf("environment variable %s: %w: %s",
-			envKey, ErrOpenVPNProtocolNotValid, protocol)
-	}
-}