Exchange `DGMulti` dispatch types for new supertypes

The idea is to replace e.g.

https://github.com/trixi-framework/Trixi.jl/blob/9a19e155e5d50c1c9811f15e00f1bc49675fed6f/src/solvers/dgmulti/flux_differencing.jl#L254

with

```julia
@inline function has_sparse_operators(::AbstractTensorProductElement,
```

This was made possible by https://github.com/jlchan/NodesAndModes.jl/pull/17

Start at src/solvers/dgmulti/flux_differencing.jl around line 254 and compare the existing DGMulti dispatch type with AbstractTensorProductElement. Read NodesAndModes.jl pull request 17 to understand the new supertypes, then update the relevant dispatches and verify that the affected solver behavior remains covered.

Written by the indexing model from the issue text.