Remove mopa dependency in turbo-tasks (#7206)
### Description
- `mopa` contains "technically" unsound code:
<https://rustsec.org/advisories/RUSTSEC-2021-0095.html>
- It hasn't been updated in 8 years, potentially indicating that it's
unmaintained.
- But it's not actually needed! We can just manually implement a method
to upcast `&dyn MagicAny` to `&dyn Any`. It just requires being more
careful in callsites about correctly upcasting before attempting to
downcast.
### Testing Instructions
I tested this by running `cargo nextest run`. I ran into some issues
with running the `node-file-trace` tests, but I think there's an issue
with my development environment (I did do the `pnpm install` step...).
All the other tests passed.
Double checked the documentation by building it with
```
RUSTDOCFLAGS="-Z unstable-options --enable-index-page" cargo doc --workspace --no-deps --keep-going --document-private-items
```
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
