Use mavenCentral() instead of jcenter()

RepositoryHandler.jcenter() was deprecated in Gradle 7.0 and removed in Gradle 9.0. The JCenter repository has redirected to Maven Central since August 2024, which the Gradle upgrade guide names as its closest direct replacement. A plain jcenter() is deleted when its repositories block already declares a plain mavenCentral(), and renamed otherwise, so that a repository with a content filter never ends up as the only way to reach artifacts it filters out. Build and settings scripts are both covered. See the Gradle upgrade guide for more information.