Deprecation warning for `_unsetindex!` for Julia 1.14 nightly

[kbarros]

On the nightly Julia builds, I'm seeing:

┌ Warning: `_unsetindex!(A, i)` is deprecated, use `Base.unsetindex!(A, i)` instead.
│   caller = popfirst!(d::DataStructures.Deque{Tuple{Float64, Vector{Int64}}}) at deque.jl:326
└ @ DataStructures ~/.julia/packages/DataStructures/IwRP2/src/deque.jl:326

Looks like Base.unsetindex! will be introduced in 1.14, specifically with DataStructures support in mind (JuliaLang/julia#58943).

AI thinks this guard would support both forms and quiet the warning:

@static if isdefined(Base, :unsetindex!)
    unsetindex!(A, i) = Base.unsetindex!(A, i)
else
    unsetindex!(A, i) = Base._unsetindex!(A, i)
end

See related discussion here: JuliaCollections/OrderedCollections.jl#172.