scylla.yaml: refresh list of experimental features
scylla.yaml: refresh list of experimental features

Our scylla.yaml contains a comment listing the available experimental
features, supposedly helping a user who might want to enable them.
I think the usefuless of this comment is dubious, but as long as we
have one, let's at least make it accurate:

* Two of the listed "experimental" features, cdc and lwt, have actually
  graduated from being experimental long ago. Although technically a user
  may still use the words "cdc" and "lwt" in the "experimental-features"
  list, doing so is pointless, and worse: This comment suggests that these
  two features are still experimental - while they are not!

* One experimental feature - alternator-ttl - is missing from this list.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20211013083247.13223-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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