Fix a concurrency bug in iterate(::Dict)

[tkf]

fix #35835

This patch simply avoids mutating the Dict struct in iterate.

An alternative (probably correct) fix is

diff --git a/base/dict.jl b/base/dict.jl
index 12ff26fbeb..c98ae28947 100644
--- a/base/dict.jl
+++ b/base/dict.jl
@@ -696,7 +696,7 @@ end
 function skip_deleted_floor!(h::Dict)
     idx = skip_deleted(h, h.idxfloor)
     if idx != 0
-        h.idxfloor = idx
+        @atomic :monotonic h.idxfloor = idx
     end
     idx
 end

but I'm posting a simpler approach since I think it makes keeping Dict implementation correct hard and drastically decreases the number of maintainers who can reason about Dict code.

[vtjnash]

Seems like the main purpose is to make pop! work in O(1) time, so we do not need it here.