Prep the CHANGELOG.md for M84

[jwren]

https://github.com/flutter/flutter-intellij/milestone/96

https://plugins.jetbrains.com/plugin/9212-flutter/edit/versions

# 84
- Fix the slow operation notification associated with PubRoot (#7885)
- Cleanup: removal of pre Dart SDK 3.0 code (#7882)
- Cleanup: removal of the deprecated Swing-- Fix the slow operation notification associated with PubRoot (#7885)based Inspector window (#7861)
- Cleanup: removal of the deprecated Outline window (#7816)
- Cleanup: removal of the deprecated Performance page window (#7816)
- Migrated all instances of EditorNotifications.Provider to the new API (#7830)

[jwren]

The kokoro-mac failure happened as this PR had not yet landed when I created this PR: #7941