infra(zip): remove CocoaPods umbrella headers.

[daymxn]

Per b/549729366,

This removes the CocoaPods umbrella headers from the final zip artifact.

With the new zip builder rewrite, we'll no longer be using CocoaPods for building our zips. As a part of this process, we won't be generating the <framework>-umbrella.h headers that CocoaPods naturally generates.

The removal of these umbrella headers is a breaking change. Although, the zip rewrite may not be done in time for the next breaking change release (Firebase 13). To avoid any breakages when it is released, this PR adds functionality to preemptively remove the CocoaPods umbrella headers. This way, our migration to the new zip builder won't have to worry about any breakage.

The changes in this PR can be split up into three work groups:

1. Update the Firebase.h umbrella header to no longer use the CocoaPods header, and instead just use the module's auto-generated -Swift header.
2. Add a post-processing step to remove the (CocoaPods) -umbrella.h headers from the final artifacts.
3. Update the module maps to use either the module's existing umbrella header (eg; FirebaseMessaging.h), or fallback to a clang umbrella directory syntax for libraries that don't expose an umbrella header (ie; umbrella ".").

DO_NOT_MERGE=Still needs a changelog entry to convey the change. Also need to do further manual testing to ensure everything works as expected.

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[danger-firebase-ios]

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