Since we can use `valkey` or `redis` clusters, should the key be simply `clusters` instead of `redis_clusters`

[nitrocode]

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Describe the Feature

The input is redis_clusters but since the engine_type can be either valkey or redis, then the input should simply be called clusters.

This can be done as a breaking change or a phased approach where var.redis_clusters is added to variables-deprecated.tf and var.clusters is added to variables.tf. Then the next major version can remove var.redis_clusters alongside any other deprecated vars.

Similar issue could plague the memcache sibling component.