[RGen] Make the transformer generator be netstandard2.1 to make edito…

…rs happy. (#22039)

In theory we can use net9.0 but some editors need netstandard2.1. We
don't want yet to move Rgen, but we do want to move the transformer one
beause it makes workign with the project easier.

src/rgen/Microsoft.Macios.Generator/TabbedStringBuilder.cs

@@ -122,6 +122,8 @@ public TabbedStringBuilder AppendLine (ReadOnlySpan<char> span)
 		return this;
 	}
 
+#if NET9_0
+
 	public TabbedStringBuilder Append (ref DefaultInterpolatedStringHandler handler)
 	{
 		WriteTabs ().Append (handler.ToStringAndClear ());
@@ -133,7 +135,7 @@ public TabbedStringBuilder AppendLine (ref DefaultInterpolatedStringHandler hand
 		WriteTabs ().Append (handler.ToStringAndClear ()).AppendLine ();
 		return this;
 	}
-
+	
 
 	/// <summary>
 	/// Append a new raw literal by prepending the correct indentation.
@@ -156,6 +158,8 @@ public TabbedStringBuilder AppendRaw (string rawString)
 
 		return this;
 	}
+
+#endif
 
 	/// <summary>
 	/// Append the generated code attribute to the current string builder. Added for convenience.

src/rgen/Microsoft.Macios.Transformer.Generator/Microsoft.Macios.Transformer.Generator/Microsoft.Macios.Transformer.Generator.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net$(BundledNETCoreAppTargetFrameworkVersion)</TargetFramework>
+        <TargetFramework>netstandard2.1</TargetFramework>
         <IsPackable>false</IsPackable>
         <Nullable>enable</Nullable>
         <LangVersion>latest</LangVersion>