3882 Update tool README for packages use (cla: yes)

3883 [in_app_purchase] Configure correct App Bundle identifier (cla: yes, p: in_app_purchase, platform-ios)

3884 Revert “Temporarily disable Windows tests” (cla: yes)

3885 Revert “Temporarily run Windows tests via GitHub Actions” (cla: yes, waiting for tree to go green)

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)

3890 Overhaul CONTRIBUTING.md (cla: yes)

3891 Begin migrating tools to NNBD (cla: yes)