Fix logging source generator CS0234 error with FormattableString namespace

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/LoggerMessageGenerator.Emitter.cs

@@ -177,7 +177,7 @@ private void GenStruct(LoggerMethod lm, string nestedIndentation)
                 string formatMethodBegin =
                     !lm.Message.Contains('{') ? "" :
                     _hasStringCreate ? "string.Create(global::System.Globalization.CultureInfo.InvariantCulture, " :
-                    "global::System.Diagnostics.CodeAnalysis.FormattableString.Invariant(";
+                    "global::System.FormattableString.Invariant(";
                 string formatMethodEnd = formatMethodBegin.Length > 0 ? ")" : "";
 
                 _builder.Append($@"