Refactor VPN configuration handling to remove device limit management

- Updated the VPN configuration commands and handlers to eliminate the device limit parameter, simplifying the configuration process.
- Adjusted related API documentation to reflect the removal of device limit management, clarifying that this setting is now handled directly in the 3x-ui by node administrators.
- Enhanced the overall codebase by removing unnecessary device limit references across various components, ensuring a cleaner and more maintainable code structure.

backend/tests/PnvPanel.IntegrationTests/TestSupport/FakeXuiPanelGateway.cs

@@ -32,7 +32,7 @@ public sealed class FakeXuiPanelGateway : IXuiPanelGateway
 
     public Task<Result<string>> AddClientAsync(
         Node node, string inboundRemoteId, VpnProtocol protocol, string clientEmail, string clientName,
-        int deviceLimit, CancellationToken cancellationToken)
+        CancellationToken cancellationToken)
         => Task.FromResult(Result.Success(Guid.NewGuid().ToString()));
 
     public Task<Result> RemoveClientAsync(
@@ -41,7 +41,7 @@ public sealed class FakeXuiPanelGateway : IXuiPanelGateway
 
     public Task<Result> UpdateClientAsync(
         Node node, string inboundRemoteId, string clientExternalId, VpnProtocol protocol,
-        string name, int deviceLimit, bool enable, CancellationToken cancellationToken)
+        string name, bool enable, CancellationToken cancellationToken)
         => Task.FromResult(Result.Success());
 
     public Task<Result<string>> BuildConnectionStringAsync(