fix Event to use normal Condition variable (#55441)

ThreadSynchronizer is only for things that are very trivial, as there
are a lot of things they are forbidden from doing (such as waiting for a
Task to set it).

Happened to notice while reviewing
#55439 (review)
that this was still using the pre-v1.2 style lock, which makes this
mostly useless in v1.4+

Author: vtjnash

base/lock.jl

@@ -498,10 +498,10 @@ This provides an acquire & release memory ordering on notify/wait.
     The `autoreset` functionality and memory ordering guarantee requires at least Julia 1.8.
 """
 mutable struct Event
-    const notify::ThreadSynchronizer
+    const notify::Threads.Condition
     const autoreset::Bool
     @atomic set::Bool
-    Event(autoreset::Bool=false) = new(ThreadSynchronizer(), autoreset, false)
+    Event(autoreset::Bool=false) = new(Threads.Condition(), autoreset, false)
 end
 
 function wait(e::Event)