Update SDK runtime version to 8.0.21 (#10128)

Author: victorisr

release-notes/8.0/8.0.21/8.0.21.md

@@ -2,7 +2,7 @@
 
 The .NET 8.0.21 and .NET SDK 8.0.415 releases are available for download. The latest 8.0 release is always listed at [.NET 8.0 Releases](../README.md).
 
-8.0 SDKs that include 8.0.20 runtimes:
+8.0 SDKs that include 8.0.21 runtimes:
 
 * [8.0.415][8.0.415]
 * [8.0.318][8.0.318]