[roll] Roll fuchsia [component_manager] TaskScope -> TaskGroup

Rebase TaskScope on fasync::TaskGroup and rename it to TaskGroup.

component_manager TaskGroup becomes a simple wrapper of
Arc<fasync::TaskGroup>.

Also, pull TaskGroup from its own library into the cm_util library.
There isn't a compelling reason to have a dedicated library for this.

Original-Fixed: b/294909925
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/905699
Original-Revision: c59b691d1c897f260f0b4a777dc895229eb93b7b
GitOrigin-RevId: cabccd98691ac2af2048cc6a8f4be893ee6858f6
Change-Id: I142c7127acf3d3e2463d1a72b2e3eb67fd1e68d6
diff --git a/stem b/stem
index 0ba1c28..e786fd3 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="ddce354d7c10775d8299c38d9443fda06a390bc7"/>
+             revision="c59b691d1c897f260f0b4a777dc895229eb93b7b"/>
   </projects>
   <hooks>
     <hook name="install-environment"