Add private preview banner

Author: cmilesb

content/operate/rc/security/manage-encryption-keys.md

@@ -8,6 +8,7 @@ categories:
 - rc
 description: Learn how to use your own keys for persistent storage encryption on Redis Cloud.
 weight: $weight
+bannerText: Self-managed encryption keys are available in Private Preview.
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 Redis Cloud databases write their data to disk whenever [persistence]({{< relref "/operate/rc/databases/configuration/data-persistence.md" >}}) is enabled.