Fix depwarn due to wrapping Vararg directly in UnionAll

[SebastianM-C]

This PR fixes depwarns introduced in #3469

cc @AayushSabharwal

Starting julia with --depwarn=error results in

Precompiling ModelingToolkit...
Info Given ModelingToolkit was explicitly requested, output will be shown live 
ERROR: LoadError: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
You may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.
Stacktrace:
 [1] UnionAll(v::Any, t::Any)
   @ Core ./boot.jl:299
 [2] top-level scope
   @ ~/.julia/packages/ModelingToolkit/r9J1w/src/systems/connectors.jl:442
 [3] include(mod::Module, _path::String)
   @ Base ./Base.jl:557
 [4] include(x::String)
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:4
 [5] top-level scope
   @ ~/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:157
 [6] include
   @ ./Base.jl:557 [inlined]
 [7] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
   @ Base ./loading.jl:2881
 [8] top-level scope
   @ stdin:6
in expression starting at /home/sebastian/.julia/packages/ModelingToolkit/r9J1w/src/systems/connectors.jl:442
in expression starting at /home/sebastian/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:1
in expression starting at stdin:6
  ✗ ModelingToolkit
  ✗ ModelingToolkit → MTKChainRulesCoreExt
  1 dependency successfully precompiled in 14 seconds. 293 already precompiled.

ERROR: The following 2 direct dependencies failed to precompile:

ModelingToolkit 

Failed to precompile ModelingToolkit [961ee093-0014-501f-94e3-6117800e7a78] to "/home/sebastian/.julia/compiled/v1.11/ModelingToolkit/jl_rqOXzf".
ERROR: LoadError: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
You may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.
Stacktrace:
 [1] UnionAll(v::Any, t::Any)
   @ Core ./boot.jl:299
 [2] top-level scope
   @ ~/.julia/packages/ModelingToolkit/r9J1w/src/systems/connectors.jl:442
 [3] include(mod::Module, _path::String)
   @ Base ./Base.jl:557
 [4] include(x::String)
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:4
 [5] top-level scope
   @ ~/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:157
 [6] include
   @ ./Base.jl:557 [inlined]
 [7] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
   @ Base ./loading.jl:2881
 [8] top-level scope
   @ stdin:6
in expression starting at /home/sebastian/.julia/packages/ModelingToolkit/r9J1w/src/systems/connectors.jl:442
in expression starting at /home/sebastian/.julia/packages/ModelingToolkit/r9J1w/src/ModelingToolkit.jl:1
in expression starting at stdin:6

[AayushSabharwal]

Thanks!