Hi Mark, afaik the Modeler version is not related to Android version compatibility.
When you publish your Android app, it's targeted at API level 23 (Android 6). You can verify this by downloading the phonegap package youself, and check the android-targetSdkVersion parameter in the /src/config.xml.mustache.
I've read that Play requires you to target your app to API level 26 (Android 8), however this requirement is due late 2018. You can try to change the android-targetSdkVersion parameter and change it from 23 to 26 and create the build in PGB.
Let us know your findings.
Edit: I just rolled out an app in Play, and also got the message. However it's just a warning message, that you need to target 26 by August 2018. So if you want you can still roll out your app targeted at 23, and ignore this message.