Remove obsolete APIs from codebase and standardize MCP diagnostic IDs (#985)

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Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>
Co-authored-by: Mackinnon Buck <mackinnon.buck@gmail.com>
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Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/ExperimentalAttribute.cs

@@ -0,0 +1,65 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NET
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>
+///  Indicates that an API is experimental and it may change in the future.
+/// </summary>
+/// <remarks>
+///   This attribute allows call sites to be flagged with a diagnostic that indicates that an experimental
+///   feature is used. Authors can use this attribute to ship preview features in their assemblies.
+/// </remarks>
+[AttributeUsage(AttributeTargets.Assembly |
+                AttributeTargets.Module |
+                AttributeTargets.Class |
+                AttributeTargets.Struct |
+                AttributeTargets.Enum |
+                AttributeTargets.Constructor |
+                AttributeTargets.Method |
+                AttributeTargets.Property |
+                AttributeTargets.Field |
+                AttributeTargets.Event |
+                AttributeTargets.Interface |
+                AttributeTargets.Delegate, Inherited = false)]
+internal sealed class ExperimentalAttribute : Attribute
+{
+    /// <summary>
+    ///  Initializes a new instance of the <see cref="ExperimentalAttribute"/> class, specifying the ID that the compiler will use
+    ///  when reporting a use of the API the attribute applies to.
+    /// </summary>
+    /// <param name="diagnosticId">The ID that the compiler will use when reporting a use of the API the attribute applies to.</param>
+    public ExperimentalAttribute(string diagnosticId)
+    {
+        DiagnosticId = diagnosticId;
+    }
+
+    /// <summary>
+    ///  Gets the ID that the compiler will use when reporting a use of the API the attribute applies to.
+    /// </summary>
+    /// <value>The unique diagnostic ID.</value>
+    /// <remarks>
+    ///  The diagnostic ID is shown in build output for warnings and errors.
+    ///  <para>This property represents the unique ID that can be used to suppress the warnings or errors, if needed.</para>
+    /// </remarks>
+    public string DiagnosticId { get; }
+
+    /// <summary>
+    ///  Gets or sets an optional message associated with the experimental attribute.
+    /// </summary>
+    /// <value>The message that provides additional information about the experimental feature.</value>
+    /// <remarks>
+    ///  This message can be used to provide more context or guidance about the experimental feature.
+    /// </remarks>
+    public string? Message { get; set; }
+
+    /// <summary>
+    ///  Gets or sets the URL for corresponding documentation.
+    ///  The API accepts a format string instead of an actual URL, creating a generic URL that includes the diagnostic ID.
+    /// </summary>
+    /// <value>The format string that represents a URL to corresponding documentation.</value>
+    /// <remarks>An example format string is <c>https://contoso.com/obsoletion-warnings/{0}</c>.</remarks>
+    public string? UrlFormat { get; set; }
+}
+#endif

src/Common/Polyfills/System/ObsoleteAttribute.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if !NET
+namespace System;
+
+/// <summary>
+/// Marks program elements that are no longer in use.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum |
+    AttributeTargets.Interface | AttributeTargets.Constructor | AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Event | AttributeTargets.Delegate,
+    Inherited = false)]
+internal sealed class ObsoleteAttribute : Attribute
+{
+    public ObsoleteAttribute()
+    {
+    }
+
+    public ObsoleteAttribute(string? message)
+    {
+        Message = message;
+    }
+
+    public ObsoleteAttribute(string? message, bool error)
+    {
+        Message = message;
+        IsError = error;
+    }
+
+    public string? Message { get; }
+
+    public bool IsError { get; }
+
+    public string? DiagnosticId { get; set; }
+
+    public string? UrlFormat { get; set; }
+}
+#endif