fix(android): replace removed jcenter() with mavenCentral()

[Michael-Betclic]

Problem

android/build.gradle declares jcenter() in both the buildscript and allprojects repository blocks. Gradle 9 removed jcenter() (it was only deprecated through Gradle 8.x), so any project that evaluates this plugin under Gradle 9 fails:

* What went wrong:
A problem occurred evaluating project ':flutter_system_proxy'.
> Could not find method jcenter() for arguments [] on repository container ...

This surfaces as soon as a consuming app upgrades its Android toolchain to Gradle 9 (e.g. via a newer Flutter SDK that regenerates the Gradle wrapper).

Fix

Replace both jcenter() calls with mavenCentral(). JCenter has been sunset (read-only since early 2022), and the artifacts these blocks need are available on Maven Central, so this is a safe, behaviour-preserving swap that also works on Gradle 8.x.

     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }

No other changes.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Rushabhshroff]

This pull request is correct, safe, and necessary.

Review Summary

Aspect	Evaluation	Details
Correctness	✅ Passed	Fixes the breaking Could not find method jcenter() exception under Gradle 9.0+.
Backward Compatibility	✅ Passed	Compatible with existing Gradle versions (6.x, 7.x, 8.x, and 9.x+).
Risk Level	🟢 Low	Standard swap; Maven Central hosts all required modern build dependencies.

Technical Breakdown

• Root Cause: JFrog officially shut down resolution services for JCenter. Gradle officially removed the jcenter() shorthand method in Gradle 9.0, turning any project calling it into a fatal build failure.
• Behavioral Impact: mavenCentral() is the standard modern replacement. Since all modern Android Gradle Plugin (AGP) and standard Flutter build tools host their dependencies on Maven Central, replacing jcenter() with mavenCentral() preserves all existing build behavior while restoring build pipeline compatibility.

Verdict: The PR should be merged as-is.