Update NuGet MCP version in Visual Studio to 1.2.3 (#7232)

Author: jeffkl

src/NuGet.Clients/NuGet.VisualStudio.Client/mcp.json

@@ -4,7 +4,7 @@
       "type": "stdio",
       "command": "dnx",
       "args": [
-        "NuGet.Mcp.Server@1.1.29",
+        "NuGet.Mcp.Server@1.2.3",
         "--source",
         "https://api.nuget.org/v3/index.json",
         "--yes"