Change name `scalar` to `density` in `LinearAdvectionEquation`

Suggestion from the [comment](https://github.com/trixi-framework/Trixi.jl/pull/1892/files#r1573387797).

"The output after line 49 still shows the variable name scalar. I guess this hard-coded in the example.
-> feature request to Trixi.jl: Please, change name `scalar` to `density`, because `scalar` just looks like a type specification :=O"

It's about how name of the variable in `LinearScalarAdvection` is shown. Take a look for example in
https://github.com/trixi-framework/Trixi.jl/blob/dd051566a9079e22dfa792b145c0b50eaafa37fd/src/equations/linear_scalar_advection_1d.jl#L26C70-L26C76.

The same for 2D and 3D.

Start in src/equations/linear_scalar_advection_1d.jl at the linked line, then find the corresponding LinearScalarAdvection definitions for 2D and 3D. Replace the displayed variable name `scalar` with `density` in all three dimensions and verify that the example output uses the new name.

Written by the indexing model from the issue text.