Feature: Add Valkey as an optional subchart to be an alternative to redis

Description of the change

Add Valkey as an optional subchart.

Benefits

It's a FOSS alternative to redis, as redis recently switched to a business source license, which could also mean that down the line, they close source more of the product. Valkey also doesn't use "slave" terminology, which has always made me uncomfortable using redis. Instead of slave, they just use "replica", but slave is still available for backwards compatibility for users switching from redis.

Possible drawbacks

It's another caching store to support, however, if parts of redis become paid, or they introduce parts of the license for redis that say you cannot get paid for support of it (like Hashicorp did with Terrform and Vault), it puts the Nextcloud company at risk for supporting it with paid support contracts as well.

Additional information

Valkey is according to their website:

an open source (BSD) high-performance key/value datastore that supports a variety of workloads such as caching, message queues, and can act as a primary database. Valkey can run as either a standalone daemon or in a cluster, with options for replication and high availability.

Valkey is available from bitnami both as stand alone valkey and Valkey cluster. I would say it would be good to support valkey cluster to allow users to scale better, but I'm open to discussion as always :)

Start by reviewing the existing Redis subchart configuration in this Helm repository and the linked Bitnami Valkey and Valkey cluster chart information. Clarify whether standalone or cluster support is expected, how it should remain optional, and which existing Redis settings need equivalent Valkey support. Done means the chart can deploy with Valkey as an alternative caching store without requiring Redis.