Address PR feedback: revert NuGet.config, use C# 14 instead of preview

Co-authored-by: jmprieur <13203188+jmprieur@users.noreply.github.com>

Author: Copilot

Directory.Build.props

@@ -39,10 +39,9 @@
     <AssemblyOriginatorKeyFile>../../build/MSAL.snk</AssemblyOriginatorKeyFile>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Nullable>enable</Nullable>
-    <!-- LangVersion set to preview to enable C# 15 extension properties for .NET 10+ AOT compatibility.
-         Extension properties are required to make CredentialDescription AOT-safe while maintaining source compatibility.
-         Once C# 15 is finalized, this should be updated to the specific version number (e.g., 15). -->
-    <LangVersion>preview</LangVersion>
+    <!-- LangVersion set to 14 to enable C# 14 extension properties for .NET 10+ AOT compatibility.
+         Extension properties are required to make CredentialDescription AOT-safe while maintaining source compatibility. -->
+    <LangVersion>14</LangVersion>
     <EnablePackageValidation>true</EnablePackageValidation>
     <PackageValidationBaselineVersion>8.0.0</PackageValidationBaselineVersion>
     <BannedApiAnalyzersVersion>4.14.0</BannedApiAnalyzersVersion>

NuGet.config

@@ -3,6 +3,5 @@
   <packageSources>
     <clear />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>

changelog.md

@@ -9,7 +9,7 @@ In practice, it's unlikely that this breaking change affects anybody as the rena
 
 ### AOT/NativeAOT Compatibility for .NET 10+
 
-Made `CredentialDescription` AOT-compatible for .NET 10+ by using C# 15 extension properties. This change:
+Made `CredentialDescription` AOT-compatible for .NET 10+ by using C# 14 extension properties. This change:
 - Removes `Certificate` and `CachedValue` as public properties from `CredentialDescription` when targeting .NET 10+
 - Adds extension properties with the same names and signatures for .NET 10+, providing property-style access
 - Maintains full source compatibility - no code changes required for consumers
@@ -19,7 +19,7 @@ Made `CredentialDescription` AOT-compatible for .NET 10+ by using C# 15 extensio
 **Technical details:**
 - For .NET 10+: `Certificate` and `CachedValue` are implemented as extension properties (not visible to config binders)
 - For older TFMs: `Certificate` and `CachedValue` remain as regular public properties
-- LangVersion updated to `preview` to enable C# 15 extension property syntax
+- LangVersion updated to `14` to enable C# 14 extension property syntax
 - Internal accessor methods (`GetCertificateInternal`, `SetCertificateInternal`, etc.) support extension properties
 
 This enhancement ensures `CredentialDescription` works seamlessly in AOT/NativeAOT compilation scenarios while maintaining backward compatibility.

src/Microsoft.Identity.Abstractions/ApplicationOptions/CredentialDescriptionExtensions.cs

@@ -12,13 +12,13 @@ namespace Microsoft.Identity.Abstractions
     /// while keeping them hidden from AOT/NativeAOT configuration binders.
     /// </summary>
     /// <remarks>
-    /// This uses C# 15 extension property syntax. The extension block defines properties that appear
+    /// This uses C# 14 extension property syntax. The extension block defines properties that appear
     /// as instance properties on CredentialDescription but are not part of the type's public API surface
     /// visible to reflection-based tools like configuration binders.
     /// </remarks>
     public static class CredentialDescriptionExtensions
     {
-        // C# 15 extension block syntax - defines extension properties on CredentialDescription
+        // C# 14 extension block syntax - defines extension properties on CredentialDescription
         extension(CredentialDescription credential)
         {
             /// <summary>