BREAKING CHANGES coming in 0.21

[brody2consult]

(updated) in followup to 0.20 (see issue #409):

• drop support for React Native pre-0.63 or pre-0.64
  (and update react-native version in generated peerDependencies)
  rationale:
  * I think it would be better to specify updated versions in peerDependencies / devDependencies / etc.
  * reduce testing burden
• possibly remove DEPRECATED --example-file-linkage option
• possibly remove experimental --write-example-podfile option
• possibly remove support for generating Maven package for Android by default (there will be an option to enable this) - see DRAFT PR DRAFT RFC: remove extra android build support #403
• possibly remove Android root build support (build outside of generated example test app) (there will be an option to enable this) - see DRAFT PR DRAFT RFC: remove extra android build support #403
• show info in log output how to disable Flipper on iOS, at least (see info in improve documentation of how to install this or other fork of React Native react-native-tvos/react-native-tvos#49 (comment)) (moving out of PR refactor: update example build & run info #427)