Prepare 7.6.0 (#18412)

* Revert "v7.5.0 release"

This reverts commit 1eafe2aebf.

* update master to v7.6.0 snapshot

* update readme

* update samples

samples/client/echo_api/csharp-restsharp/.openapi-generator/VERSION

@@ -1 +1 @@
-7.5.0-SNAPSHOT
+7.6.0-SNAPSHOT

samples/client/echo_api/csharp-restsharp/README.md

@@ -6,7 +6,7 @@ This C# SDK is automatically generated by the [OpenAPI Generator](https://openap
 
 - API version: 0.1.0
 - SDK version: 1.0.0
-- Generator version: 7.5.0-SNAPSHOT
+- Generator version: 7.6.0-SNAPSHOT
 - Build package: org.openapitools.codegen.languages.CSharpClientCodegen
 
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