Remove reference to `jcenter()` repository

**Context**

[JCenter shutdown impact on Gradle builds.](https://blog.gradle.org/jcenter-shutdown)
[On February 3 2021, JFrog announced that they will be shutting down Bintray and JCenter.](https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/)
