Remove redis 'experimental' tag

[datengraben]

Why is there an experimental Tag?
Are there problems in any real life deployments?
The implementation of the RedisAdapter and the FilebasedAdapter are part of the symfony lib, we use only the interface.

I've got it running on my local machine.

1. I had to install either the redis extension or predis/predis composer package.
2. Using Wordpress via wp-env I had to add the redis container to my wordpress network after I spun up redis-stack image.

@hansmorb So maybe we just add one of package/extension to composer.json and remove experimental tag?