Btrfs graph driver is not experimental

Btrfs graph driver is not experimental now according to #4838 and CHANGELOG.md is saying "btrfs is no longer considered experimental.".

Docker-DCO-1.1-Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com> (github: ichik1)
Merge pull request #7367 from ichik1/btrfs-is-not-experimental

Btrfs graph driver is not experimental