[firebase_core] Updated documentation for FirebaseOptions.apiKey

[pszklarska]

Description

Due to changes in the newest version of firebase-ios-sdk plugin, now FirebaseOptions.apiKey is required also on iOS. Not passing this property in the Dart code results with an error:

Could not configure Firebase Installations due to invalid FirebaseApp options. The following parameters are nil or empty: `FirebaseOptions.APIKey`

Link to the change in iOS plugin: https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseInstallations/Source/Library/FIRInstallations.m#L127
Link to the PR with change: firebase/firebase-ios-sdk#4683

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[Salakar]

Hey @pszklarska - thank you for sending this contribution 🎉 . Our firebase_core rework has now landed on master via #2890 which unfortunately now supersedes this PR - we hope to start publishing preview releases to test out next week.