fix(firestore, iOS): change firebase-ios-sdk dependency from 'from' to 'exact' version in multiple Package.swift files

[SelaseKay]

Description

This is related to #18319.

Even though we specify firebase-ios-sdk version 12.13.0, Swift Package Manager still resolves it to 12.14.0 because we’re using from in the package declaration:

.package(url: "https://github.com/firebase/firebase-ios-sdk", from: firebase_sdk_version)

With from, SPM is allowed to pick the latest compatible minor version. In this case, it resolved to 12.14.0, which introduced an interface change and ended up breaking the build.

Using exact instead pins the dependency strictly to the specified version, preventing any automatic upgrades.
Reference: https://docs.swift.org/package-manager/PackageDescription/PackageDescription.html

Related Issues

Fixes #18319

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