Stop Tracking RDS Engine Version

## Description

We have identified that some RDS clusters are using older engine versions that are no longer supported.
This patch will help use stop tracking the `EngineVersion` property in our templates and let AWS manage the engine version without our intervention.

## Impact

There is no downtime expected from this change, and it should not affect the operation of the RDS clusters.

## Changes

- Removed `EngineVersion` property from the RDS cluster resource.
- Update engine version to the latest supported version

## If You Accept

- We will stop tracking the `EngineVersion` property in our templates.
- Your RDS cluster will have the latest supported engine version.