Further fix to configuration classes using ISet, resolving regression with custom 404 pages (#19573)

Further fix to configuration classes using ISet, resolving regression with custom 404 pages.

Author: AndyButland

src/Umbraco.Core/CompatibilitySuppressions.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- https://learn.microsoft.com/dotnet/fundamentals/package-validation/diagnostic-ids -->
+<Suppressions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Umbraco.Cms.Core.Configuration.Models.ContentSettings.get_Error404Collection</Target>
+    <Left>lib/net9.0/Umbraco.Core.dll</Left>
+    <Right>lib/net9.0/Umbraco.Core.dll</Right>
+    <IsBaselineSuppression>true</IsBaselineSuppression>
+  </Suppression>
+</Suppressions>

src/Umbraco.Core/Configuration/Models/ContentSettings.cs

@@ -50,7 +50,7 @@ public class ContentSettings
     /// <summary>
     ///     Gets or sets a value for the collection of error pages.
     /// </summary>
-    public ISet<ContentErrorPage> Error404Collection { get; set; } = new HashSet<ContentErrorPage>();
+    public IEnumerable<ContentErrorPage> Error404Collection { get; set; } = [];
 
     /// <summary>
     ///     Gets or sets a value for the preview badge mark-up.

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Configuration/Models/Validation/ContentSettingsValidatorTests.cs

@@ -42,9 +42,9 @@ private static ContentSettings BuildContentSettings(string culture = "en-US", st
             new ContentSettings
             {
                 Error404Collection =
-                {
+                [
                     new() { Culture = culture, ContentId = 1 },
-                },
+                ],
                 Imaging =
                 {
                     AutoFillImageProperties =