feat(vpn): auto detection of IPv6 support

- `OPENVPN_IPV6` removed
- Affects OpenVPN
- Use the same mechanism for OpenVPN and Wireguard
- Check only once at program start since this is unlikely to change at runtime
- Log if IPv6 is supported
- Remove `IPv6` boolean from settings structs
- Move IPv6 detection as a method on NetLinker

internal/provider/provider.go

@@ -13,7 +13,7 @@ import (
 // Provider contains methods to read and modify the openvpn configuration to connect as a client.
 type Provider interface {
 	GetConnection(selection settings.ServerSelection) (connection models.Connection, err error)
-	OpenVPNConfig(connection models.Connection, settings settings.OpenVPN) (lines []string)
+	OpenVPNConfig(connection models.Connection, settings settings.OpenVPN, ipv6Supported bool) (lines []string)
 	Name() string
 	PortForwarder
 	FetchServers(ctx context.Context, minServers int) (