Migrate or remove all API usages that will be removed with time

[jwren]

Many of the errors reported in the JetBrains marketplace, https://plugins.jetbrains.com/plugin/9212-flutter/edit/versions/stable/615584, are for deprecated issues which will be removed, these usages in the Flutter plugin need to be migrated or removed.

[jwren]

The pre-requisite for this was to migrate to Gradle IntelliJ platform 2.1.x to have the updated verifier which is done, this work will not be finished for this upcoming Milestone, bumping to the next one.