Pin dask to avoid 2024.8.0 - problems with masked fill/missing values

[valeriupredoi]

Description

Closes #2503

~I will also open an issue at Dask with an MRC, but this is all we can do, for now, at our end~

So far, this is an iris issue reported here https://github.com/SciTools/iris/issues/6109

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.71%. Comparing base (9636505) to head (8439463).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2504 +/- ## ======================================= Coverage 94.71% 94.71% ======================================= Files 249 249 Lines 14123 14123 ======================================= Hits 13376 13376 Misses 747 747 ```

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[valeriupredoi]

gents @bouweandela @schlunma shall we merge this until the good folk at iris figure out how to fix the issue? :beer: