[📚] Update messaging install instructions to remove ios background modes

[mikehardy]

I think this is leftovers from when we had a background modes used for advanced local notifications - this feature has since been removed from React Native - so I think the docs should be updated to reflect this now

Originally posted by @Salakar in https://github.com/firebase/firebase-ios-sdk/discussions/8977#discussioncomment-1805400

Further context, the original confusion:

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Does the Background Modes entitlement need to be added for an iOS app to use Firebase Cloud Messaging, RemoteConfig, etc.?

Google's official Firebase iOS SDK documentation only says we need to add the Push Notifications entitlement.

However the React Native Firebase website says:

Next the "Background Modes" capability needs to be enabled, along with both the "Background fetch" and "Remote notifications" sub-modes.

Which of these two is correct?

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[github-actions[bot]]

Hello 👋, to help manage issues we automatically close stale issues.

This issue has been automatically marked as stale because it has not had activity for quite some time.Has this issue been fixed, or does it still require attention?

This issue will be closed in 15 days if no further activity occurs.

Thank you for your contributions.