chore: Prepare release 2.0.0

[mparticle-automation]

Preparing for release 2.0.0

• Updated pubspec.yaml to new version
• Updated CHANGELOG.md
• Updated VERSION file
• Ran a dry-run publish

[cursor]

PR Summary

Low Risk
Release-metadata-only changes (version bumps and changelog/link updates) with no runtime code modifications; risk is limited to potential packaging/versioning mistakes.

Overview
Prepares the 2.0.0 release by bumping the package version in pubspec.yaml and VERSION.

Updates CHANGELOG.md with the 2.0.0 release notes (including noted breaking iOS requirements/dependency changes) and adjusts the compare links so [unreleased] now targets 2.0.0 → HEAD.

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