`dplyr` compatibility

This warning now appears:

```r
## Warning: Returning more (or less) than 1 row per `summarise()` group was deprecated in
## dplyr 1.1.0.
## ℹ Please use `reframe()` instead.
## ℹ When switching from `summarise()` to `reframe()`, remember that `reframe()`
## always returns an ungrouped data frame and adjust accordingly.
## ℹ The deprecated feature was likely used in the epipredict package.
```

In various parts of the package, we `summarise()` epi_dfs. First encountered in https://cmu-delphi.github.io/epipredict/articles/preprocessing-and-models.html

Task: examine the examples and vignettes to find where this warning is thrown. Track down the recommended fixes and implement them (likely in various parts of the package).