Flutter example generated as part of plugin generated from ffi_plugin template fails to run with older version of flutter.
So it needs to have lower-bound constraint generated in its pubspec.yaml that matches flutter that is used to generate the project.

Otherwise, user could encounter various compile-time errors when running generated example with older version of flutter.

$ flutter create --template plugin_ffi --platforms linux,windows p
...
$ cat p/example/pubspec.yaml
...
 environment:
  sdk: '>=2.18.0-283.0.dev <3.0.0'
...
$

cc @stuartmorgan