MPI: Replace deprecated methods

[ranocha]

When running CI, we get warnings like

┌ Warning: `Gatherv(sendbuf::AbstractArray, counts::Vector{Cint}, root::Integer, comm::Comm)` is deprecated, use `Gatherv!(view(sendbuf, 1:counts[MPI.Comm_rank(comm) + 1]), if Comm_rank(comm) == root
│         VBuffer(similar(sendbuf, sum(counts)), counts)
│     else
│         nothing
│     end, root, comm)` instead.
│   caller = (::Trixi.var"#486#487"{Float64,Float64,Int64,Trixi.TreeMesh{2,Trixi.ParallelTree{2}},Trixi.CompressibleEulerEquations2D{Float64},Trixi.DG{Float64,Trixi.LobattoLegendreBasis{Float64,4,Array{Float64,2},StaticArrays.SArray{Tuple{4,2},Float64,2,8},StaticArrays.SArray{Tuple{4,4},Float64,2,16}},Trixi.LobattoLegendreMortarL2{Float64,4,StaticArrays.SArray{Tuple{4,4},Float64,2,16}},typeof(Trixi.flux_lax_friedrichs),Trixi.VolumeIntegralWeakForm},NamedTuple{(:elements, :interfaces, :mpi_interfaces, :boundaries, :mortars, :mpi_cache, :fstar_upper_threaded, :fstar_lower_threaded),Tuple{Trixi.ElementContainer2D{Float64,4,3},Trixi.InterfaceContainer2D{Float64,4,3},Trixi.MPIInterfaceContainer2D{Float64,4,3},Trixi.BoundaryContainer2D{Float64,4,3},Trixi.L2MortarContainer2D{Float64,4,3},Trixi.MPICache,Array{StaticArrays.MArray{Tuple{4,4},Float64,2,16},1},Array{StaticArrays.MArray{Tuple{4,4},Float64,2,16},1}}},Dict{Symbol,Any},Array{Int32,1},Array{Int32,1}})(::HDF5.HDF5File) at save_solution_dg.jl:134
└ @ Trixi ~/work/Trixi.jl/Trixi.jl/src/callbacks/save_solution_dg.jl:134

┌ Warning: `Gatherv(sendbuf::AbstractArray, counts::Vector{Cint}, root::Integer, comm::Comm)` is deprecated, use `Gatherv!(view(sendbuf, 1:counts[MPI.Comm_rank(comm) + 1]), if Comm_rank(comm) == root
│         VBuffer(similar(sendbuf, sum(counts)), counts)
│     else
│         nothing
│     end, root, comm)` instead.
│   caller = save_solution_file(::Array{Float64,4}, ::Float64, ::Float64, ::Int64, ::Trixi.TreeMesh{2,Trixi.ParallelTree{2}}, ::Trixi.CompressibleEulerEquations2D{Float64}, ::Trixi.DG{Float64,Trixi.LobattoLegendreBasis{Float64,4,Array{Float64,2},StaticArrays.SArray{Tuple{4,2},Float64,2,8},StaticArrays.SArray{Tuple{4,4},Float64,2,16}},Trixi.LobattoLegendreMortarL2{Float64,4,StaticArrays.SArray{Tuple{4,4},Float64,2,16}},typeof(Trixi.flux_lax_friedrichs),Trixi.VolumeIntegralWeakForm}, ::NamedTuple{(:elements, :interfaces, :mpi_interfaces, :boundaries, :mortars, :mpi_cache, :fstar_upper_threaded, :fstar_lower_threaded),Tuple{Trixi.ElementContainer2D{Float64,4,3},Trixi.InterfaceContainer2D{Float64,4,3},Trixi.MPIInterfaceContainer2D{Float64,4,3},Trixi.BoundaryContainer2D{Float64,4,3},Trixi.L2MortarContainer2D{Float64,4,3},Trixi.MPICache,Array{StaticArrays.MArray{Tuple{4,4},Float64,2,16},1},Array{StaticArrays.MArray{Tuple{4,4},Float64,2,16},1}}}, ::Trixi.SaveSolutionCallback, ::Dict{Symbol,Any}; system::String) at s...
└ @ Trixi ~/work/Trixi.jl/Trixi.jl/src/callbacks/save_solution_dg.jl:107

┌ Warning: `Allgather!(sendrecvbuf, count::Integer, comm::Comm)` is deprecated, use `Allgather!(UBuffer(sendrecvbuf, count), comm)` instead.
│   caller = create_cache(::Trixi.TreeMesh{2,Trixi.ParallelTree{2}}, ::Trixi.CompressibleEulerEquations2D{Float64}, ::Trixi.DG{Float64,Trixi.LobattoLegendreBasis{Float64,4,Array{Float64,2},StaticArrays.SArray{Tuple{4,2},Float64,2,8},StaticArrays.SArray{Tuple{4,4},Float64,2,16}},Trixi.LobattoLegendreMortarL2{Float64,4,StaticArrays.SArray{Tuple{4,4},Float64,2,16}},typeof(Trixi.flux_lax_friedrichs),Trixi.VolumeIntegralWeakForm}, ::Type{T} where T) at dg_2d_parallel.jl:124
└ @ Trixi ~/work/Trixi.jl/Trixi.jl/src/solvers/dg/dg_2d_parallel.jl:124
┌ Warning: `Allgather!(sendrecvbuf, count::Integer, comm::Comm)` is deprecated, use `Allgather!(UBuffer(sendrecvbuf, count), comm)` instead.
│   caller = create_cache(::Trixi.TreeMesh{2,Trixi.ParallelTree{2}}, ::Trixi.CompressibleEulerEquations2D{Float64}, ::Trixi.DG{Float64,Trixi.LobattoLegendreBasis{Float64,4,Array{Float64,2},StaticArrays.SArray{Tuple{4,2},Float64,2,8},StaticArrays.SArray{Tuple{4,4},Float64,2,16}},Trixi.LobattoLegendreMortarL2{Float64,4,StaticArrays.SArray{Tuple{4,4},Float64,2,16}},typeof(Trixi.flux_lax_friedrichs),Trixi.VolumeIntegralWeakForm}, ::Type{T} where T) at dg_2d_parallel.jl:124
└ @ Trixi ~/work/Trixi.jl/Trixi.jl/src/solvers/dg/dg_2d_parallel.jl:124

See https://github.com/trixi-framework/Trixi.jl/pull/292/checks?check_run_id=1390251395