feat(env): rename vpn port forwarding variables

- `VPN_PORT_FORWARDING_STATUS_FILE`
- `VPN_PORT_FORWARDING`
- Deprecate PIA specific variables for VPN port forwarding

Dockerfile

@@ -108,8 +108,8 @@ ENV VPN_SERVICE_PROVIDER=pia \
     OWNED_ONLY=no \
     # # Private Internet Access only:
     PRIVATE_INTERNET_ACCESS_OPENVPN_ENCRYPTION_PRESET= \
-    PRIVATE_INTERNET_ACCESS_VPN_PORT_FORWARDING=off \
-    PRIVATE_INTERNET_ACCESS_VPN_PORT_FORWARDING_STATUS_FILE="/tmp/gluetun/forwarded_port" \
+    VPN_PORT_FORWARDING=off \
+    VPN_PORT_FORWARDING_STATUS_FILE="/tmp/gluetun/forwarded_port" \
     # # Cyberghost only:
     OPENVPN_CERT= \
     OPENVPN_KEY= \