update 1.5 Readme with new v2 version 2.2.20

Author: frantuma

README.md

@@ -33,7 +33,8 @@ The OpenAPI Specification has undergone several revisions since initial creation
 
 Swagger core Version      | Release Date | OpenAPI Spec compatibility | Notes | Status
 ------------------------- | ------------ | -------------------------- | ----- | ----
-2.2.19 (**current stable**)| 2023-11-10   | 3.x           | [tag v2.2.19](https://github.com/swagger-api/swagger-core/tree/v2.2.19) | Supported
+2.2.20 (**current stable**)| 2023-12-19   | 3.x           | [tag v2.2.20](https://github.com/swagger-api/swagger-core/tree/v2.2.20) | Supported
+2.2.19                     | 2023-11-10   | 3.x           | [tag v2.2.19](https://github.com/swagger-api/swagger-core/tree/v2.2.19) | Supported
 2.2.18                     | 2023-10-25   | 3.x           | [tag v2.2.18](https://github.com/swagger-api/swagger-core/tree/v2.2.18) | Supported
 2.2.17                     | 2023-10-12   | 3.x           | [tag v2.2.17](https://github.com/swagger-api/swagger-core/tree/v2.2.17) | Supported
 2.2.16                     | 2023-09-18   | 3.x           | [tag v2.2.16](https://github.com/swagger-api/swagger-core/tree/v2.2.16) | Supported