status/network: Title bluetooth connections as "Tether"

As per the latest mockups, Bluetooth network connections are simply
labeled "Tether". Override _getDefaultName in NMBluetoothToggle
and always return "Tether" as title.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2619>

js/ui/status/network.js

@@ -1836,6 +1836,11 @@ class NMBluetoothToggle extends NMDeviceToggle {
             'gnome-network-panel.desktop');
     }
 
+    _getDefaultName() {
+        // Translators: "Tether" from "Bluetooth Tether"
+        return _('Tether');
+    }
+
     _createDeviceMenuItem(device) {
         return new NMBluetoothDeviceItem(this._client, device);
     }