fix(macos)!: macOS build failures (fixes #4628)

[Salakar]

Description

• Adds missing file symlinks that weren't picked up before merging feat(cloud_firestore): Move Snapshot handling into a EventChannel #4209 as CI was disabled (this PR also re-enables it to see if the issue has resolved itself).
• Bumps all example apps to explicitly target osx 10.12.
• Adds a quick ruby script to each macOS podspec to give a better log output with solutions on how to fix for users who are not on 10.12 or later deployment target for osx (Flutter default is currently 10.11) (note I just did a quick 'equals to' check on the version just to cover the most common case for ease).

Related Issues

Fixes #4628

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[github-actions]

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https://flutter-firebase-docs--pr4634-salakar-firestore-a-fqg5avib.web.app

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[mikehardy]

I left some nits on the repetitiveness of the ruby, seems like could be firebase_core functionality to honor DRY principle, but things look reasonable. Having bite-size commits made it easy to roll through for review +1 on that