Prepare v7.8.0 (#19048)

* Revert "v7.7.0 release"

This reverts commit f3dd32385e.

* prepare v7.8.0

* update readme

* update doc

* update samples

samples/server/petstore/spring-boot-nullable-set/.openapi-generator/VERSION

@@ -1 +1 @@
-7.7.0-SNAPSHOT
+7.8.0-SNAPSHOT

samples/server/petstore/spring-boot-nullable-set/src/main/java/org/openapitools/api/NullableApi.java

@@ -1,5 +1,5 @@
 /**
- * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech) (7.7.0-SNAPSHOT).
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech) (7.8.0-SNAPSHOT).
  * https://openapi-generator.tech
  * Do not edit the class manually.
  */
@@ -32,7 +32,7 @@ import java.util.Map;
 import java.util.Optional;
 import javax.annotation.Generated;
 
-@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", comments = "Generator version: 7.7.0-SNAPSHOT")
+@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", comments = "Generator version: 7.8.0-SNAPSHOT")
 @Validated
 @Tag(name = "nullable", description = "the nullable API")
 public interface NullableApi {

samples/server/petstore/spring-boot-nullable-set/src/main/java/org/openapitools/model/ObjectWithUniqueItems.java

@@ -28,7 +28,7 @@ import javax.annotation.Generated;
  * ObjectWithUniqueItems
  */
 
-@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", comments = "Generator version: 7.7.0-SNAPSHOT")
+@Generated(value = "org.openapitools.codegen.languages.SpringCodegen", comments = "Generator version: 7.8.0-SNAPSHOT")
 public class ObjectWithUniqueItems {
 
   @Valid