Update README to reflect .NET support

Author: k-wojcik

README.md

@@ -17,7 +17,7 @@ A .NET wrapper for [CRoaring](https://github.com/RoaringBitmap/CRoaring) - a C i
 |        **osx**        |                    | :white_check_mark: |                    | :white_check_mark: |
 
 ### Supported frameworks
-* .NET 8
+* .NET 8+
 * .NET 6
 
 ## Usage