Avoid broadcasting (`.=`) in favor of `@threaded` ?

I got reminded of this by https://github.com/trixi-framework/Trixi.jl/issues/2283

See for instance https://github.com/trixi-framework/Trixi.jl/pull/924

I also observe that
```julia
@threaded for i in eachindex(u)
u[i] = # Do stuff
end
```

is usually faster than
```julia
u .= # Do stuff
```

Unfortunately this broadcasting is used at many places:

https://github.com/search?q=repo%3Atrixi-framework%2FTrixi.jl+.%3D+path%3A%2F%5Esrc%5C%2F%2F&type=code

Review the linked Trixi.jl issue 2283 and pull request 924 first, then inspect the many `.=` uses under `src/` identified by the repository search. Determine which broadcasts are suitable for `@threaded` and how correctness and performance should be checked; done requires an agreed scope and validated replacements rather than a blanket change.

Written by the indexing model from the issue text.