Update Codegen to support Gradle 9

[paulbakker]

While testing with a newly created Spring Boot project I noticed the following warning:

com.netflix.dgs.codegen`
    - [warn]  The org.gradle.api.plugins.Convention type has been deprecated.`com.netflix.dgs.codegen`
This is scheduled to be removed in Gradle 9.0.
The org.gradle.api.plugins.Convention type has been deprecated.
        - The org.gradle.api.plugins.Convention type has been deprecated.`the-org-gradle-api-plugins-convention-type-has-been-deprecated`

[paulbakker]

Fixed https://github.com/Netflix/dgs-codegen/releases/tag/v8.0.0