Update broker to allow customers to upgrade to gp3

[markdboyd]

In order to allow customers to benefit from gp3 volumes for storage, we want to update this broker to allow customers to self-upgrade from gp2 to gp3 volumes

Background on gp2 vs gp3: https://aws.amazon.com/ebs/general-purpose/#:~:text=gp3%20offers%20SSD%2Dperformance%20at,improving%20performance%20and%20reducing%20costs.

Acceptance Criteria

• Manual testing demonstrates that a customer can upgrade from a gp2 to gp3 storage volumes for their database

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Security considerations

None

Implementation sketch

• Add support in database instance updates for changing storage volume from gp2 to gp3
• Add unit test for updating from gp2 to gp3