Update deprecated SDK version + extension version on website

- [x] update `config.isUnsupportedSoon` to return `!versionIsAtLeast(this.version, "3.0.0")` instead of `false`
- [ ] ~~Update the website to note which extension versions deprecate and remove support for older SDKs~~ https://github.com/Dart-Code/Dart-Code/issues/5325