Switch from an explicit default to O+C for redis readReplicasMode (#5899) (#11420)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/5899.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+redis: fixed an issue where older redis instances had a dangerous diff on the field `read_replicas_mode`, adding a default of `READ_REPLICAS_DISABLED`. Now, if the field is not set in config, the value of the field will keep the old value from state.
+```

google/resource_redis_instance.go

@@ -285,6 +285,7 @@ resolution and up to nine fractional digits.`,
 			},
 			"read_replicas_mode": {
 				Type:         schema.TypeString,
+				Computed:     true,
 				Optional:     true,
 				ForceNew:     true,
 				ValidateFunc: validateEnum([]string{"READ_REPLICAS_DISABLED", "READ_REPLICAS_ENABLED", ""}),
@@ -293,8 +294,7 @@ If not set, Memorystore Redis backend will default to READ_REPLICAS_DISABLED.
 - READ_REPLICAS_DISABLED: If disabled, read endpoint will not be provided and the 
 instance cannot scale up or down the number of replicas.
 - READ_REPLICAS_ENABLED: If enabled, read endpoint will be provided and the instance 
-can scale up and down the number of replicas. Default value: "READ_REPLICAS_DISABLED" Possible values: ["READ_REPLICAS_DISABLED", "READ_REPLICAS_ENABLED"]`,
-				Default: "READ_REPLICAS_DISABLED",
+can scale up and down the number of replicas. Possible values: ["READ_REPLICAS_DISABLED", "READ_REPLICAS_ENABLED"]`,
 			},
 			"redis_configs": {
 				Type:     schema.TypeMap,

website/docs/r/redis_instance.html.markdown

@@ -313,7 +313,6 @@ The following arguments are supported:
   instance cannot scale up or down the number of replicas.
   - READ_REPLICAS_ENABLED: If enabled, read endpoint will be provided and the instance 
   can scale up and down the number of replicas.
-  Default value is `READ_REPLICAS_DISABLED`.
   Possible values are `READ_REPLICAS_DISABLED` and `READ_REPLICAS_ENABLED`.
 
 * `region` -