chore: regenerate sttp4 samples after upstream merge

Author: plaflamme

samples/client/petstore/scala-sttp4-circe/.openapi-generator/VERSION

@@ -1 +1 @@
-7.20.0-SNAPSHOT
+7.21.0-SNAPSHOT

samples/client/petstore/scala-sttp4-circe/README.md

@@ -2,7 +2,7 @@
 
 OpenAPI Petstore
 - API version: 1.0.0
-    - Generator version: 7.20.0-SNAPSHOT
+    - Generator version: 7.21.0-SNAPSHOT
 
 This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.