I recently ran into this issue myself. Here is my Q&A to my findings and solution. Google Play Store blocking app update due to Billing Library version AIDL and must update to at least version 6.0.1

And for reference, below is the solution.

In the AndroidManifest was the following:

<uses-permission android:name="com.android.vending.BILLING" />

Removing the above from the manifest, resolved the issue.