Migrate from Ant to Maven or Gradle
Ant is alright as a toolbox for a smaller project, but offers little in the way of dependency management, reproducible builds and tests, and standardisation of development and deployment processes. All three issues are important to solve with the project growing more complex and under active development.
Apache Maven and Gradle are Ant's major successors (with Gradle arguably as a successor of Maven). Both offer dependency management and lifecycle management (compilation, testing, building a jar, etc.).
Based on the little experience of both tools I've had in the past, both following a convention-over-configuration paradigm, where the most important parts of lifecycle and dependency management are available out-of-the-box and straightforward to use. Please see an overview of the tools, as well as their main differences, in the comments.
Do we want to migrate from Ant to Maven or Gradle? What are our considerations?