Bump Firebase iOS SDK (12.18.0 → 12.19.0)

[SelaseKay]

Source: Firebase SDK watcher workflow run

React Native Firebase's pinned Firebase iOS SDK is behind the latest release.

	Version
Current pin (packages/app/package.json sdkVersions.ios.firebase)	12.18.0
Latest	12.19.0

Release notes: https://firebase.google.com/support/release-notes/ios

This issue tracks the pin only. Wrapper API work from the 12.19.0 notes window is out of scope for the bump PR.

[SelaseKay]

Bump PR #9316 (12.18.0 → 12.19.0).