Prep the CHANGELOG.md for M84 (#7942)

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)
```

Author: jwren

CHANGELOG.md

@@ -1,3 +1,11 @@
+# 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-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)
+
 # 83
 - First version for Meerkat, Android Studio 2024.3 (#7799)
 - Message in the Flutter Outline window that the window is now deprecated (#7778)

resources/META-INF/plugin.xml

@@ -44,6 +44,15 @@
 
   <change-notes>
     <![CDATA[
+<h1>84</h1>
+<ul>
+  <li>Fix the slow operation notification associated with PubRoot (#7885)</li>
+  <li>Cleanup: removal of pre Dart SDK 3.0 code (#7882)</li>
+  <li>Cleanup: removal of the deprecated Swing-based Inspector window (#7861)</li>
+  <li>Cleanup: removal of the deprecated Outline window (#7816)</li>
+  <li>Cleanup: removal of the deprecated Performance page window (#7816)</li>
+  <li>Migrated all instances of EditorNotifications.Provider to the new API (#7830)</li>
+</ul>
 <h1>83</h1>
 <ul>
   <li>First version for Meerkat, Android Studio 2024.3 (#7799)</li>