Burstable SKU is not able to support major version upgrade due to the performance limitation of this SKU.
If you need to perform a major version upgrade on your Azure Database for MySQL Flexible Server instance and are currently using Burstable SKU, one temporary solution would be to upgrade to General Purpose or Business Critical SKU, perform the upgrade, and then switch back to Burstable SKU.
Upgrading to a higher SKU might involve a change in pricing and might result in increased costs for your deployment. However, since the upgrade process is not expected to take a long time, the added costs shouldn't be significant.