Adopt wg-async as a subteam (WG) of lang (#1639)

We're working to move things out from under the launching-pad team to
more appropriate places.  It makes the most sense for the async
working group to be adopted as a subteam of lang, so let's do that.

Note that there are ongoing discussions of how we on lang might want
to further refactor this working group.  This PR doesn't seek to
resolve those.  It just puts the working group in our domain.  We can
later adjust as we deem proper.

teams/wg-async.toml

@@ -1,5 +1,5 @@
 name = "wg-async"
-subteam-of = "launching-pad"
+subteam-of = "lang"
 kind = "working-group"
 
 [people]