Some plugin READMEs have outdated version support

For example, image_picker README lists iOS 12:
https://github.com/flutter/packages/blob/18b9cc5f2b0820cb6164f0150958ba76db75c61a/packages/image_picker/image_picker/README.md?plain=1#L10

But the minimum is iOS 13: https://github.com/flutter/packages/pull/10231

All plugins READMEs should be audited for correctness for all platforms.