Prepare 7.9.0 snapshot (#19386)

* Revert "v7.8.0 release (#19385)"

This reverts commit 6bdc452f92.

* update samples for v7.9.0-snapshot

samples/client/petstore/csharp/unityWebRequest/standard2.0/Petstore/.openapi-generator/VERSION

@@ -1 +1 @@
-7.8.0-SNAPSHOT
+7.9.0-SNAPSHOT

samples/client/petstore/csharp/unityWebRequest/standard2.0/Petstore/README.md

@@ -6,7 +6,7 @@ This C# SDK is automatically generated by the [OpenAPI Generator](https://openap
 
 - API version: 1.0.0
 - SDK version: 1.0.0
-- Generator version: 7.8.0-SNAPSHOT
+- Generator version: 7.9.0-SNAPSHOT
 - Build package: org.openapitools.codegen.languages.CSharpClientCodegen
 
 <a id="version-support"></a>