Move module to `assertj/assertj-generator`

[scordio]

• The project should become a submodule of assertj/assertj-generator and renamed to assertj-generator-maven-plugin
• The Maven artifact should be relocated to the new Maven coordinates
• All open issues should be transferred to the new repository
• All open pull requests should be evaluated
• The README should be updated to reflect the transfer

Lastly, the repository should be archived.