doc(aws_elasticache_cluster): add valkey option as engine

[remiflament]

#39745

Add missing valkey option for aws_elasticache_cluster engine in the documentation

[github-actions]

Community Note

Voting for Prioritization

• Please vote on this pull request by adding a 👍 reaction to the original post to help the community and maintainers prioritize this pull request.
• Please see our prioritization guide for information on how we prioritize.
• Please do not leave "+1" or other comments that do not add relevant new information or questions, they generate extra noise for issue followers and do not help prioritize the request.

For Submitters

• Review the contribution guide relating to the type of change you are making to ensure all of the necessary steps have been taken.
• For new resources and data sources, use skaff to generate scaffolding with comments detailing common expectations.
• Whether or not the branch has been rebased will not impact prioritization, but doing so is always a welcome surprise.

[remiflament]

Related bug in progress: #39905

[bryantbiggs]

Note: Elasticache clusters do not support ValKey, only replication groups #39972 (comment)

This PR can be closed for now since the docs are correct