This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.

Under android/ directory, ignore:
GeneratedPluginRegistrant.java

Under ios/ directory, ignore:
GeneratedPluginRegistrant.h
GeneratedPluginRegistrant.m
Pods/

* Revert "Revert "Don't try to resolve CocoaPods unless project has plugins" (#9719)"

This reverts commit f739e9e0.

* Remove channels integration test’s partial cocoapods artifacts

* Mark channels_integration_test_ios flaky

* Removed personal dev team from ios code siging

* Removed provisioning profile diffs against other samples