Change deprecated aesthetic

[We have](https://r-quantities.github.io/quantities/articles/introduction.html):

```r
p0 + geom_errors()
#> Warning: Using the `size` aesthetic in this geom was deprecated in ggplot2 3.4.0.
#> ℹ Please use `linewidth` in the `default_aes` field and elsewhere instead.
```