Add .NET 9 to the version support matrix

Author: bkoelman

README.md

@@ -85,13 +85,16 @@ See also our [versioning policy](./VERSIONING_POLICY.md).
 |                   |              | 7        | 7                     |
 | 5.5+              | Stable       | 6        | 6, 7                  |
 |                   |              | 7        | 7                     |
-|                   |              | 8        | 8                     |
+|                   |              | 8        | 8, 9                  |
+|                   |              | 9        | 9                     |
 | master            | Preview      | 6        | 6, 7                  |
 |                   |              | 7        | 7                     |
-|                   |              | 8        | 8                     |
+|                   |              | 8        | 8, 9                  |
+|                   |              | 9        | 9                     |
 | openapi           | Experimental | 6        | 6, 7                  |
 |                   |              | 7        | 7                     |
-|                   |              | 8        | 8                     |
+|                   |              | 8        | 8, 9                  |
+|                   |              | 9        | 9                     |
 
 ## Contributing