deprecate(lmxproxy): move all LmxProxy code, tests, and docs to deprecated/

LmxProxy is no longer needed. Moved the entire lmxproxy/ workspace, DCL
adapter files, and related docs to deprecated/. Removed LmxProxy registration
from DataConnectionFactory, project reference from DCL, protocol option from
UI, and cleaned up all requirement docs.

src/ScadaLink.DataConnectionLayer/DataConnectionOptions.cs

@@ -13,7 +13,4 @@ public class DataConnectionOptions
 
     /// <summary>Timeout for synchronous write operations to devices.</summary>
     public TimeSpan WriteTimeout { get; set; } = TimeSpan.FromSeconds(30);
-
-    /// <summary>LmxProxy keep-alive interval for gRPC sessions.</summary>
-    public TimeSpan LmxProxyKeepAliveInterval { get; set; } = TimeSpan.FromSeconds(30);
 }