Remove use_modular_headers! from Podfiles (#7796)
â ï¸ _This PR does not update packages' versions as it only affects unpublished parts of example apps and tests._
We added use_modular_headers! to our Podfiles as we originally planned to phase out use_frameworks! (see https://github.com/flutter/flutter/pull/42204). However, our plans have now changed and we are instead phasing out CocoaPods entirely in favor of Swift Package Manager.
CocoaPods's use_frameworks! and use_modular_headers! are two different overlapping options that should not be used together. This change removes the use_modular_headers! from example apps' Podfiles. This change does not affect packages themselves.
Part of https://github.com/flutter/flutter/issues/156259