fix(health): use TCP dialing instead of ping

- `HEALTH_TARGET_ADDRESS` to replace `HEALTH_ADDRESS_TO_PING`
- Remove `github.com/go-ping/ping` dependency
- Dial TCP the target address, appending `:443` if port is not set

Dockerfile

@@ -124,7 +124,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     LOG_LEVEL=info \
     # Health
     HEALTH_SERVER_ADDRESS=127.0.0.1:9999 \
-    HEALTH_ADDRESS_TO_PING=github.com \
+    HEALTH_TARGET_ADDRESS=github.com:443 \
     HEALTH_VPN_DURATION_INITIAL=6s \
     HEALTH_VPN_DURATION_ADDITION=5s \
     # DNS over TLS