Remove deprecated jcenter repo

[lukas-vlcek]

How to replicate the warning:

./gradlew clean check --warning-mode all

Build file '/Users/lukas.vlcek/projects/prometheus-exporter/build.gradle': line 91
The RepositoryHandler.jcenter() method has been deprecated. This is scheduled to be removed in Gradle 9.0. JFrog announced JCenter's sunset in February 2021. Use mavenCentral() instead. Consult the upgrading guide for further information: https://docs.gradle.org/8.5/userguide/upgrading_version_6.html#jcenter_deprecation
        at build_bhirhg0ive4hpm14ov25a3i3c$_run_closure2.doCall$original(/Users/lukas.vlcek/projects/prometheus-exporter/build.gradle:91)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
        at build_bhirhg0ive4hpm14ov25a3i3c.run(/Users/lukas.vlcek/projects/prometheus-exporter/build.gradle:85)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)

[lukas-vlcek]

We already make use of mavenCentral() so just deleting the line should be fine.