Add persistence_iam_identity to Redis Instance

[modular-magician]

Fixes #6850 . I also updated all the examples to use Redis 4.0 as it is the default one Google creates now.

Release Note Template for Downstream PRs (will be copied)

redis: added `persistence_iam_identity` output field to `google_redis_instance`

redis: explicitly noted in `google_redis_instance` documentation that `"REDIS_5_0"` is supported

Derived from GoogleCloudPlatform/magic-modules#3805

[ghost]

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