add date metadata to C# 13 proposals

Author: BillWagner

docfx.json

@@ -500,6 +500,7 @@
                 "_csharplang/proposals/csharp-10.0/*.md": "08/07/2021",
                 "_csharplang/proposals/csharp-11.0/*.md": "09/30/2022",
                 "_csharplang/proposals/csharp-12.0/*.md": "08/15/2023",
+                "_csharplang/proposals/csharp-13.0/*.md": "10/31/2024",
                 "_csharplang/proposals/*.md": "10/31/2024",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md": "11/08/2022",
                 "_roslyn/docs/compilers/CSharp/Compiler Breaking Changes - DotNet 8.md": "09/26/2023",