chore(firebase_remote_config): migrate to use new firebase_core.

[Salakar]

Description

• Depend on new firebase_core (pre-release).
• Firebase iOS SDK versions are now locked to use the same version defined in
  firebase_core.
• Firebase Android SDK versions are now using the Firebase Bill of Materials (BoM)
  to specify individual SDK versions. BoM version is also sourced from
  firebase_core.
• Added support for MacOS. (Add macOS support #1653)
• Allow iOS & MacOS plugins to be imported as modules.

Related Issues

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Checklist

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• 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

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• 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.

Local Testing

Android & iOS:

MacOS: