Please consider making rsyncable non-experimental

**Is your feature request related to a problem? Please describe.**
I'd like to make use of zstd rsyncable mode, without having to rely on experimental library features.

**Describe the solution you'd like**
I'd love to have zstd rsyncable mode marked non-experimental (assuming it is, in fact, non-experimental).

(https://github.com/facebook/zstd/issues/3153 may need to get addressed first.)