Create breaking change guide for Android minSdkVersion increase to 21

[parlough]

A page and an entry in https://docs.flutter.dev/release/breaking-changes would be helpful now that the change has landed in main.

These entries help users be aware of upcoming changes and potentially prepare for them.

[parlough]

@zanderso or @dnfield Could one of you write this up?

It'll likely doesn't need to be too long or detailed. Perhaps a little background on the what and the why. Then instructions on what, if anything, developers need to do to make their apps to be compatible with the change or to benefit from it.

Let me know if I should redirect this to someone else or if you have any questions. Thanks :D