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.

[ridik-il]

Thanks @Kunalbehbud — clean, well-scoped change and I appreciate the thorough test coverage (the dedicated Update test asserting Engine reaches ModifyReplicationGroup is exactly right, and updating all the cacheClusterNeedsUpdate fixtures was the correct call). I verified the whole provider builds against the bumped SDK and all elasticache tests pass, and confirmed removing +immutable is safe (it was advisory — no webhook/test enforced it — and engine is required so existing resources are unaffected). One thing to fix before merge: go.sum isn't tidy (go mod tidy drops 20 stale lines, including hashes for versions newer than what go.mod pins) — please run go mod tidy and commit. Optionally, consider an Enum=redis;valkey validation on the now-mutable engine field and a doc note that only redis→valkey migration is supported. Happy to approve once go.sum is sorted.

[Kunalbehbud]

Rebased onto master. The only conflict was in managed_test.go — a declaration collision with the LogDeliveryConfiguration/authTokenSecretRef helpers added in #2276, so I kept both sides and left the feature and the Engine migration test untouched. CI should be green again; ready to merge whenever convenient, and happy to rebase again if it drifts before you get to it.