Resolve open Firestore Codable Transition issues

[paulb777]

Description

Resolve the unresolved issues when merging #9465

Reproducing the issue

No response

Firebase SDK Version

10.0.0

Xcode Version

n/a

Installation Method

Swift Package Manager

Firebase Product(s)

Firestore

Targeted Platforms

iOS

Relevant Log Output

No response

If using Swift Package Manager, the project's Package.resolved

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If using CocoaPods, the project's Podfile.lock

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[andrewheard]

@morganchen12 I replied on the original PR. Most important ones are 1 and 2.