feat(elasticache): support inplace migration to valkey engine

[Kunalbehbud]

Fixes #2290

This PR adds support for in-place ElastiCache ReplicationGroup migration from Redis to Valkey.

It makes spec.forProvider.engine mutable, updates the ElastiCache client logic to pass Engine through ModifyReplicationGroupInput, and extends update detection so engine changes are reconciled correctly. It also updates the AWS SDK dependency to a version that exposes the ElastiCache Valkey support needed by this flow.

Checklist

• Added unit tests covering engine propagation and update detection
• Added controller test verifying ModifyReplicationGroupInput includes Engine
• Regenerated CRDs successfully
• Ran make test successfully

[Kunalbehbud]

Leaving a ping here in case this fell through the cracks. Happy to adjust the Valkey migration behavior or test coverage if there are concerns.