chore(docs): mark pact-net 4.x as deprecated

Users are advised to migrate to pact-net 5.x

https://github.com/pact-foundation/pact-net/blob/master/docs/upgrading-to-5.md

Author: YOU54F

README.md

@@ -248,7 +248,7 @@ Due to using a shared native library instead of C# for the main Pact logic only
 | Version | Status     | [Spec] Compatibility | Install            |
 | ------- | ---------- | -------------------- | ------------------ |
 | 5.x     | Stable     | 2, 3, 4              | See [installation] |
-| 4.x     | Stable     | 2, 3                 | See [installation] |
+| 4.x     | Deprecated | 2, 3                 | See [installation] |
 | 3.x     | Deprecated | 2                    |                    |
 
 ## Roadmap