[in_app_purchase_storekit] Make Storekit 2 the default

[LouiseHsu]

Make Storekit 2 the default. Fixes flutter/flutter#159871

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[hellohuanlin]

I saw you requested review but it's still "Draft" mode. is it ready to review?

[hellohuanlin]

should this be 0.4.0 instead? iirc breaking change should be major bump, but this is still 0. version, so a minor bump?
(cc @stuartmorgan-g for advice)

[stuartmorgan-g]

Yes, breaking changes need a "major" (which for pre-1.0 is the second number, per Dart convention) version bump.