Replace jcenter() with mavenCentral() in build.gradle

[antonlicht]

This pull request updates the Gradle build configuration to improve repository management and future compatibility. The main change is replacing the deprecated jcenter() repository with mavenCentral() in both the main and example Android build files.

Repository updates:

• Replaced jcenter() with mavenCentral() in the buildscript.repositories section of android/build.gradle to use a more reliable and supported repository.
• Replaced jcenter() with mavenCentral() in the rootProject.allprojects.repositories section of android/build.gradle for consistency across all projects.
• Updated buildscript.repositories in example/android/build.gradle to use mavenCentral() instead of jcenter().
• Updated allprojects.repositories in example/android/build.gradle to use mavenCentral() instead of jcenter().