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/echo_api/powershell/.openapi-generator/VERSION

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

samples/client/echo_api/powershell/README.md

@@ -6,7 +6,7 @@ This PowerShell module is automatically generated by the [OpenAPI Generator](htt
 
 - API version: 0.1.0
 - SDK version: 0.1.2
-- Generator version: 7.8.0-SNAPSHOT
+- Generator version: 7.9.0-SNAPSHOT
 - Build package: org.openapitools.codegen.languages.PowerShellClientCodegen
 
 <a id="frameworks-supported"></a>

samples/client/echo_api/powershell/src/PSOpenAPITools/en-US/about_PSOpenAPITools.help.txt

@@ -11,7 +11,7 @@ LONG DESCRIPTION
 
     - API version: 0.1.0
     - SDK version: 0.1.2
-    - Generator version: 7.8.0-SNAPSHOT
+    - Generator version: 7.9.0-SNAPSHOT
     - Build package: org.openapitools.codegen.languages.PowerShellClientCodegen
 
     Frameworks supported: