Swap Base.Event to use a ThreadSynchronizer (#51093)

and make some of its other fields const.

Author: gbaraldi

base/lock.jl

@@ -435,10 +435,10 @@ 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
-    notify::Threads.Condition
-    autoreset::Bool
+    const notify::ThreadSynchronizer
+    const autoreset::Bool
     @atomic set::Bool
-    Event(autoreset::Bool=false) = new(Threads.Condition(), autoreset, false)
+    Event(autoreset::Bool=false) = new(ThreadSynchronizer(), autoreset, false)
 end
 
 function wait(e::Event)