feat(dotnet): add [Obsolete] GithubToken property for backward compatibility

Adds a deprecated GithubToken property that forwards to GitHubToken,
allowing existing consumers to migrate without breaking changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/Types.cs

@@ -74,6 +74,16 @@ protected CopilotClientOptions(CopilotClientOptions? other)
     /// </summary>
     public string? GitHubToken { get; set; }
 
+    /// <summary>
+    /// Obsolete. Use <see cref="GitHubToken"/> instead.
+    /// </summary>
+    [Obsolete("Use GitHubToken instead.", error: false)]
+    public string? GithubToken
+    {
+        get => GitHubToken;
+        set => GitHubToken = value;
+    }
+
     /// <summary>
     /// Whether to use the logged-in user for authentication.
     /// When true, the CLI server will attempt to use stored OAuth tokens or gh CLI auth.