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.

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