Migrate maven repository from jcenter to mavenCentral

[kelBurke]

Includes https://github.com/ened/flutter_isolate/tree/spawned-in-isolate

[BetterB0y]

can we merge this @rmawatson?
currently, the jcenter is down, and it's better if we can fix this on flutter_isolate.
thank you

[BetterB0y]

@nmfisher Can you look into it?

[nmfisher]

For future reference, jcenter has been deprecated, hence why this change was suggested:

JFrog, the company that maintains the JCenter artifact repository used by many Android projects, made JCenter a read-only repository on March 31st, 2021. According to [the announcement](https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/), JCenter will allow downloads of existing artifacts indefinitely.

Developers who publish artifacts on JCenter should migrate their packages to a new host, such as [Maven Central](https://maven.apache.org/repository/index.html).

https://developer.android.com/studio/build/jcenter-migration