V15: Ensure elements cache is cleared on subscribers in load balanced scenarios (#19128)

* Clear elementscache from cache refreshers

* Add very simple test ensuring the elements cache is cleared

---------

Co-authored-by: Kenn Jacobsen <kja@umbraco.dk>

tests/Umbraco.Tests.Integration/Umbraco.Core/Cache/DistributedCacheRefresherTests.cs

@@ -0,0 +1,51 @@
+using NUnit.Framework;
+using Umbraco.Cms.Core.Cache;
+using Umbraco.Cms.Core.Services.Changes;
+using Umbraco.Cms.Infrastructure.HybridCache;
+using Umbraco.Cms.Tests.Common.Testing;
+using Umbraco.Cms.Tests.Integration.Testing;
+
+namespace Umbraco.Cms.Tests.Integration.Umbraco.Core.Cache;
+
+// We need to make sure that it's the distributed cache refreshers that refresh the elements cache
+// see: https://github.com/umbraco/Umbraco-CMS/issues/18467
+[TestFixture]
+[UmbracoTest(Database = UmbracoTestOptions.Database.NewSchemaPerFixture)]
+internal sealed class DistributedCacheRefresherTests : UmbracoIntegrationTest
+{
+    private IElementsCache ElementsCache => GetRequiredService<IElementsCache>();
+
+    private ContentCacheRefresher ContentCacheRefresher => GetRequiredService<ContentCacheRefresher>();
+
+    private MediaCacheRefresher MediaCacheRefresher => GetRequiredService<MediaCacheRefresher>();
+
+    [Test]
+    public void DistributedContentCacheRefresherClearsElementsCache()
+    {
+        var cacheKey = "test";
+        PopulateCache("test");
+
+        ContentCacheRefresher.Refresh([new ContentCacheRefresher.JsonPayload()]);
+
+        Assert.IsNull(ElementsCache.Get(cacheKey));
+    }
+
+    [Test]
+    public void DistributedMediaCacheRefresherClearsElementsCache()
+    {
+        var cacheKey = "test";
+        PopulateCache("test");
+
+        MediaCacheRefresher.Refresh([new MediaCacheRefresher.JsonPayload(1, Guid.NewGuid(), TreeChangeTypes.RefreshAll)]);
+
+        Assert.IsNull(ElementsCache.Get(cacheKey));
+    }
+
+    private void PopulateCache(string key)
+    {
+        ElementsCache.Get(key, () => new object());
+
+        // Just making sure something is in the cache now.
+        Assert.IsNotNull(ElementsCache.Get(key));
+    }
+}