The answer to this question also, partly, needs to take into consideration what parts of Android you want to take advantage of in your application. The question of what min version Flutter supports has been answered here a couple of times so I won't answer that, but the Android support libraries will also need specific min versions.
If you plan to use plugins of any kind then you will probably hit multiple issues if your min version is too low. Do you need Firebase? Do you need specific camera functions?
Google also just announced required bumps for min versions with regards to Google Play that you should review.
My advice is to follow documentation, analytics and best practices to determine what you "true" min version should be.