AAD B2C support notice for articles

Author: guardrex

aspnetcore/blazor/includes/azure-active-directory-b2c-support-notice.md

@@ -0,0 +1,7 @@
+<!-- UPDATE 15.0 - Remove this INCLUDE file and delete all content
+                   on B2C when .NET 15 releases in 2030, which is 
+                   when B2C support ends for existing customer
+                   accounts established prior to 5/1/25. -->
+
+> [!NOTE]
+> Azure Active Directory B2C is no longer available as a service to new customers as of May 1, 2025. AAD B2C tenants are supported for customers with accounts established prior to May 1, 2025 until 2030. For more information, see [Azure AD B2C: Frequently asked questions (FAQ)](/azure/active-directory-b2c/faq).