make SyntaxAttribute internal

Author: linkdotnet

src/LinkDotNet.StringBuilder/StringSyntaxAttribute.cs

@@ -4,7 +4,7 @@ namespace System.Diagnostics.CodeAnalysis;
 /// <summary>Fake version of the StringSyntaxAttribute, which was introduced in .NET 7</summary>
 [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Field | AttributeTargets.Property)]
 [SuppressMessage("Design", "CA1019:Define accessors for attribute arguments", Justification = "The sole purpose is to have the same public surface as the class in .NET7 and above.")]
-public sealed class StringSyntaxAttribute : Attribute
+internal sealed class StringSyntaxAttribute : Attribute
 {
     /// <summary>The syntax identifier for strings containing composite formats.</summary>
     public const string CompositeFormat = nameof(CompositeFormat);