Prompt users to upgrade flutter skills when they are out of date

Flutter's officially supported IDEs (VS Code & IntelliJ / Android Studio) should prompt users to upgrade their skills when they are stale. This is similar to how the IDE prompts users to run `flutter pub get` when their dependencies have changed or how the IDE notifies users that a new flutter release is available.

This should be possible since the skills have a last modified field and the IDE can fetch information about the most up to date skills to compare the dates. Additional information is needed from @jwren or @johnpryan about how the IDEs should lookup the most up to date skills version.

CC @jwren @johnpryan @mariamhas