3887 [in_app_purchase] update min Flutter version to 1.20.0 (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-ios)

3896 [in_app_purchase] Add reference to the codelab in the README (cla: yes, p: in_app_purchase)

3897 [in_app_purchase] Expose the NSLocale object (cla: yes, p: in_app_purchase, platform-ios)

3916 [in_app_purchase] Migrate maven repo from jcenter to mavenCentral (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-android)