Add guide to migrate Redis to Valkey

Author: mortenlj

docs/persistence/redis/how-to/migrate-to-valkey.md

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+---
+tags: [valkey, redis, how-to, migrate]
+---
+
+# Migrate to Valkey
+
+If you need to keep the data and your application needs to be available during the migration, reach out to the nais team for assistance.
+
+## Prerequisites
+
+- Your app uses Redis for a simple caching store and can handle the data missing at startup.
+
+## Variants
+
+The steps to migrate differ depending on how your Redis instance has been created:
+
+1. You created the Redis instance implicitly by adding a `redis` block to your application manifest.
+2. You created the Redis instance explicitly by creating a `Redis` resource.
+
+### Implicit Redis instance
+
+If you created the Redis instance implicitly by adding a `redis` block to your application manifest, follow these steps:
+
+1. Copy the `redis` block in your application manifest to use `valkey` instead. The fields are the same.
+2. Deploy the updated application manifest.
+
+   Your application is now using Valkey instead of Redis.
+3. Prepare Redis for deletion:
+
+   `kubectl patch redis redis-<TEAM-NAME>-<INSTANCE-NAME> --type json -p='[{"op": "replace", "path": "/spec/terminationProtection", "value": false}]'`
+
+4. Remove the `redis` block from your application manifest and deploy the updated manifest.
+
+   The Redis instance will now be deleted.
+5. Congrats! Your application now uses Valkey instead of Redis.
+
+### Explicit Redis instance
+
+If you created the Redis instance explicitly by creating a `Redis` resource, follow these steps:
+
+1. Make a copy of your Redis manifest to hold your Valkey manifest.
+2. Update `kind` from `Redis` to `Valkey`.
+3. Change the name from `redis-<TEAM-NAME>-<INSTANCE-NAME>` to `valkey-<TEAM-NAME>-<INSTANCE-NAME>`.
+4. Deploy the manifest.
+5. For each application that uses this Redis instance:
+   1. Change the `redis` block in the application manifest to use `valkey` instead. The fields are the same.
+   2. Deploy the updated application manifest.
+6. Delete the Redis manifest from source control so it doesn't get recreated.
+7. Delete the old Redis instance: `kubectl delete redis redis-<TEAM-NAME>-<INSTANCE-NAME>`.
+8. Congrats! Your application now uses Valkey instead of Redis.