init: Warn about future deprecation of memcg.swappiness

This memcg feature is not supported in v2, which is now the default.
While memcg v1 is still currently supported, we don't think there are
any existing users which depend on this swappiness behavior. Let's warn
about it's future deprecation now.

Bug: 393020526
Change-Id: Ia2c6a77cb88bb97248e395ce9d60fd81af5384e8