docs: Add 2.0.0 migration guide and changelog entry

[denischilik]

Background

Prepares docs for the upcoming 2.0.0 release: an [Unreleased] entry in CHANGELOG.md and a standalone MIGRATING.md covering the Apple SDK 9 upgrade on iOS. Docs-only — no code changes.

What Has Changed

• Added MIGRATING.md with 1.x → 2.0.0 upgrade steps: iOS 15.6 deployment target, CocoaPods dependency change (mParticle-Apple-SDK/mParticle ~> 8.5 → mParticle-Apple-SDK ~> 9.0; subspecs removed in SDK 9), new MPRoktEvents event types, and the new iOS-only Rokt.selectShoppableAds Dart API.
• Added an [Unreleased] block to CHANGELOG.md listing the breaking changes and new Rokt features.
• For native-only changes in the underlying iOS SDK (DB reset, AppDelegateProxy removal, ATS / regional routing, Obj-C/Swift-level Rokt renames), the guide links to the Apple SDK migration document instead of duplicating it.

Screenshots/Video

N/A — docs only.

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Additional Notes

No Dart source changes. flutter analyze and flutter test were run locally after the changes and are unchanged from main.

[cursor]

PR Summary

Low Risk
Docs-only changes; no runtime behavior or dependencies are modified in this PR.

Overview
Documents the upcoming 2.0.0 release by adding an [Unreleased] changelog section and a new MIGRATING.md covering the 1.x → 2.0 upgrade path.

The docs call out iOS breaking changes (minimum deployment target raised to 15.6 and CocoaPods dependency change to mParticle-Apple-SDK ~> 9.0 with subspec removal), and describe new/updated Rokt integration details including the iOS-only selectShoppableAds API and four additional MPRoktEvents event types/payload keys.

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