dplyr `summarise()` to `reframe()` migration

This warning becomes gradually more prominently, first in tests, now when running the Shiny app:

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 gsDesign package.
  Please report the issue at
  <https://github.com/keaven/gsDesign/issues>.
This warning is displayed once every 8 hours.
Call `lifecycle::last_lifecycle_warnings()` to see where this
warning was generated.

See dplyr 1.1.0 blog post about the context of the changes needed.

Impacted code:

summarise() is used in R/gsqplot.R.
summarize() is also imported but probably should not have been. It's used in Poisson mixture vignette.

keaven / gsDesign

dplyr `summarise()` to `reframe()` migration #114