Update README.md

Updated the content to include Umbraco 15 and .NET 9

Author: Aaen

15/umbraco-cms/fundamentals/setup/upgrading/README.md

@@ -58,6 +58,7 @@ Use the table below to determine which .NET version to upgrade to when going thr
 
 | CMS version | .NET version |
 | ----------- | ------------ |
+| 15          | 9.0          |
 | 14          | 8.0          |
 | 13          | 8.0          |
 | 12          | 7.0          |