TST: update expected dtype for sum of decimals with pyarrow 21+
This should fix the failure we started having for the pyarrow nightly build (behaviour change in https://github.com/apache/arrow/pull/44184 to increase the precision of the resulting decimal for sum)
Apologies for going quickly here, but going to merge this to have green CI on 2.3.x for releasing Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 2.3.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -x -m1 ebca3c56c1f9454ef5d2de5bb19ce138e1619504
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #61799: TST: update expected dtype for sum of decimals with pyarrow 21+'
```

4. Push to a named branch:

```
git push YOURFORK 2.3.x:auto-backport-of-pr-61799-on-2.3.x
```

5. Create a PR against branch 2.3.x, I would have named this PR:

> "Backport PR #61799 on branch 2.3.x (TST: update expected dtype for sum of decimals with pyarrow 21+)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 Manual backport -> https://github.com/pandas-dev/pandas/pull/61801