docs(messaging): update usage documentation with modular code examples

[Seth-jo]

Description

This PR updates the usage section of the Cloud Messaging documentation examples to use the modular API instead of the deprecated namespaced API.

The documentation currently uses the namespaced API in all its examples. With the incoming deprecation of the namespaced API, usage of the modular API is the better practice. As such, the documentation should reflect this.

I found it difficult uplifting to Expo SDK 53 and updating my code without examples to utilise - just using the Firebase docs does not feel as intuitive as examples from the actual implementation.

Within the PR, I just updated the current examples in Usage to use the modular API.

Related issues

Fixes #8710

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • [X] Yes
• My change supports the following platforms;
  • [X] Android
  • [X] iOS
  • [ ] Other (macOS, web)
• My change includes tests;
  • [ ] e2e tests added or updated in packages/\*\*/e2e
  • [ ] jest tests added or updated in packages/\*\*/__tests__
• [ ] I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • [ ] Yes
  • [X] No

Test Plan

This is only a change to the documentation - no functional changes were made.

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