Prepare 7.12.0 (#20512)

* Revert "v7.11.0 release (#20508)"

This reverts commit a7240eeefe.

* prepare 7.12.0 snapshot version

* update samples

* update readme

samples/client/echo_api/r/.openapi-generator/VERSION

@@ -1 +1 @@
-7.11.0-SNAPSHOT
+7.12.0-SNAPSHOT

samples/client/echo_api/r/README.md

@@ -7,7 +7,7 @@ This API client was generated by the [OpenAPI Generator](https://openapi-generat
 
 - API version: 0.1.0
 - Package version: 1.0.0
-- Generator version: 7.11.0-SNAPSHOT
+- Generator version: 7.12.0-SNAPSHOT
 - Build package: org.openapitools.codegen.languages.RClientCodegen
 
 ## Installation