`jacobian_ad_forward` Not working with `IndicatorHennemannGassner`

Not sure if this is a known issue, but right now calling `jacobian_ad_forward` with a semidiscretization build from `VolumeIntegralShockCapturingHG` with shock indicator [`IndicatorHennemannGassner`](https://github.com/trixi-framework/Trixi.jl/blob/3ea50d1d108d9c3b4d5ce463a40a20bfa7937633/src/solvers/dgsem_tree/indicators_1d.jl) like in [`examples/structured_1d_dgsem/elixir_euler_sedov.jl`](https://github.com/trixi-framework/Trixi.jl/blob/3ea50d1d108d9c3b4d5ce463a40a20bfa7937633/examples/structured_1d_dgsem/elixir_euler_sedov.jl#L58) gives the error

```
ERROR: MethodError: no method matching Float64(::ForwardDiff.Dual{Nothing, Float64, 12})
Closest candidates are:
(::Type{T})(::Real, ::RoundingMode) where T<:AbstractFloat at rounding.jl:200
(::Type{T})(::T) where T<:Number at boot.jl:772
(::Type{T})(::VectorizationBase.Double{T}) where T<:Union{Float16, Float32, Float64, VectorizationBase.Vec{<:Any, <:Union{Float16, Float32, Float64}}, VectorizationBase.VecUnroll{var"#s36", var"#s35", var"#s34", V} where {var"#s36", var"#s35", var"#s34"<:Union{Float16, Float32, Float64}, V<:Union{Bool, Float16, Float32, Float64, Int16, Int32, Int64, Int8, UInt16, UInt32, UInt64, UInt8, SIMDTypes.Bit, VectorizationBase.AbstractSIMD{var"#s35", var"#s34"}}}} at ~/.julia/packages/VectorizationBase/MOxkH/src/special/double.jl:100
...
Stacktrace:
[1] convert(#unused#::Type{Float64}, x::ForwardDiff.Dual{Nothing, Float64, 12})
@ Base ./number.jl:7
[2] setindex!(A::Vector{Float64}, x::ForwardDiff.Dual{Nothing, Float64, 12}, i1::Int64)
@ Base ./array.jl:966
[3] macro expansion
@ ~/.julia/dev/Trixi/src/solvers/dgsem_tree/indicators_1d.jl:51 [inlined]

...
```

i.e., what goes wrong is that the datatypes within

[`indicator[i] = indicator_hg.variable(u_local, equations)`](https://github.com/trixi-framework/Trixi.jl/blob/3ea50d1d108d9c3b4d5ce463a40a20bfa7937633/src/solvers/dgsem_tree/indicators_1d.jl#L51)

disagree.

Start with src/solvers/dgsem_tree/indicators_1d.jl at the indicator[i] assignment around line 51, then reproduce the failure using examples/structured_1d_dgsem/elixir_euler_sedov.jl with jacobian_ad_forward. Trace the IndicatorHennemannGassner path and verify that the indicator storage and ForwardDiff result use compatible datatypes.

Written by the indexing model from the issue text.