A dependabot upgrade to change the com.android.billingclient:billing version from 5.2.0 to 6.0.0 created deprecation warnings that needed to be suppressed.
We should remove usages of the deprecated BillingFlowParams.ProrationMode and then remove the warning suppressions.
See the PR that introduced the suppressions flutter/packages#3988, as well as the documentation for the deprecated class (which includes a suggestion on what to replace with).