JCenter repository will be shut down, and is already read-only.

### Checklist

- [ ] the issue is indeed a bug and not a support request
- [ ] issue doesn't already exist: https://github.com/kivy/python-for-android/issues
- [ ] I have a short, runnable example that reproduces the issue
- [ ] I reproduced the problem with the latest development version (`p4a.branch = develop`)
- [ ] I used the grave accent (aka backticks) to format code or logs when appropriated

### Versions

- Python: *
- OS: *
- Kivy: *
- Cython: *
- OpenJDK: *

### Description

We need to remove `jcenter()` from our `build.tmpl.gradle` and add an alternative (as `mavenCentral`)

Open build.tmpl.gradle and locate the repository declarations containing jcenter(). Replace the unavailable repository with the stated alternative, then run the relevant Android packaging or build checks to confirm dependencies still resolve.

Written by the indexing model from the issue text.