Build warnings

[bes89]

Warning 1

Please remove usages of `jcenter()` Maven repository from your build scripts and migrate your build to other Maven repositories.
This repository is deprecated and it will be shut down in the future.
See http://developer.android.com/r/tools/jcenter-end-of-service for more information.
Currently detected usages in: root project 'android', project ':app', project ':countdown_flutter', ...
Affected Modules: app

Warning 2

Using flatDirs should be avoided because it doesn't support any meta-data formats.
Currently detected usages:
- repository flatDir used in: root project 'android', project ':app', project ':countdown_flutter', ...
Affected Modules: app

To Reproduce
Steps to reproduce the behavior:

1. Build with Android Studio 4.2 and see warning in logs

Expected behavior
This warning should not be shown.