Clean up SDK version + add callout

Author: DanTup

sdk-version-compatibility.md

@@ -12,9 +12,15 @@ table th, table td {
 
 The table below indicates the latest Dart/Flutter extensions that can be used when using older versions of the Dart/Flutter SDKs.
 
+{:.callout.title}
+### Dart 2.19 / Flutter 3.7 Support Ending Soon
+
+{:.callout.body}
+Support for Dart SDK v2.19 and earlier (and the corresponding Flutter 3.7 and earlier) is ending soon. Please update to at least Dart 3.0 / Flutter 3.10 to avoid disruption.
+
 | Dart SDK | Flutter SDK | Extension Version | |
 | ----- | ----- | ----- | ----- |
-| up to v3.0 | up to v3.10 | Support ending soon <!-- <span title="v?.??+1 removes legacy analysis server protocol support">Supported until v?.??</span> --> | [announcement](https://groups.google.com/g/flutter-announce/c/JQHzM3FbBGI) |
+| up to v2.19 | up to v3.7 | Support ending soon <!-- <span title="v?.??+1 removes legacy analysis server protocol support">Supported until v?.??</span> --> | [announcement](https://groups.google.com/g/flutter-announce/c/JQHzM3FbBGI) |
 | v3.0 - current | v3.10 - current | Currently supported |
 
 To switch to a specific version of the extensions, use the **Install Specific Version...** from the cog menu on the extension in VS Code's extensions side bar: