Remove JCenter lines from build.gradle or at least specify what used by it directly

[emanuelb]

Jcenter declared in file build.gradle

bitcoin-kit-android/build.gradle

Line 7 in 38611c8

jcenter()

bitcoin-kit-android/build.gradle

Line 21 in 38611c8

jcenter()

Jcenter service is closed (it is available only in read-only mode, no updates for packages will be made)
more details on needed migration:
https://docs.gradle.org/7.6.1/userguide/upgrading_version_6.html#jcenter_deprecation
https://developer.android.com/studio/build/jcenter-migration
https://blog.gradle.org/jcenter-shutdown

and here content on how to migrate:
https://jeroenmols.com/blog/2021/02/04/migratingjcenter/

probably one or both of below entries are needed instead of jcenter usage:

        gradlePluginPortal()
        mavenCentral()

You can declare specific usages of jcenter like explained in https://jeroenmols.com/blog/2021/02/04/migratingjcenter/#consumed-dependencies-from-bintrayjcenter and like it's done for example in this PR of unstoppable wallet "Gradually migrate from jcenter to mavenCentral" (in case jcenter usage is still required)
https://github.com/horizontalsystems/unstoppable-wallet-android/pull/3752/files