fatal error: ‘Flutter/Flutter.h’ file not found

After switching the flutter channel to beta and back to stable, my app did not compile anymore. The compilation stopped with the following error:

fatal error: 'Flutter/Flutter.h' file not found

Multiple flutter clean and channel switches did not work in this case.

The following commands fixed this behavior:

rm ios/Flutter/Flutter.podspec
flutter clean

See: https://github.com/flutter/flutter/issues/70895#issuecomment-744734693