[roll] Roll fuchsia [fuchsia-async] Remove WeakScopeRef, Join::detach()

As the documentation explains, the type is unnecessary because it can't
be used to create cycles that didn't already exist. A ScopeRef can be
held by a user's task code, but each Task already holds a ScopeRef, so
it would just be redundant.

WeakScopeRef does prevent cycles in the internal ownership graph.

Testing this also exposed a flaw in the Join API. The intention was to
allow cancelling or detaching after polling the Join future, but Join is
not Unpin, so those methods must accept `Pin<&mut Self>` to be useful.
Admittedly this is a clunky API to use. I fixed cancel() but removed
detach() since implementing it would require putting the inner scope
in an Option and for the rare cases that would ever need it, mem::drop()
is a way out.

Original-Bug: 339724492
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1152733
Original-Revision: fe0477726529afe531f46ad709390d1bc7f035b7
GitOrigin-RevId: d0bb73591d0ab2a669bfd60bc9d3e57f99a1964f
Change-Id: I6825192a0a6bc199ef9bc58fa8d5090a8c6bfd11

Author: Tyler Mandry

stem

@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="b84e00e46f0a0132fd9dfb9a535084c23c56adeb"/>
+             revision="fe0477726529afe531f46ad709390d1bc7f035b7"/>
   </projects>
   <hooks>
     <hook name="install-environment"