Upkeep for ggplot2 4.0.0: `<ggplot> + x` instead of `<ggplot> %+% x`

[hfrick]

Loads of these now popping up:

Warning ([test-prob-gain_curve.R:245:3](vscode-file://vscode-app/Applications/Positron.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html#)): gain_curve() with case weights scales `.n` and `.n_events`
<ggplot> %+% x was deprecated in ggplot2 4.0.0.
i Please use <ggplot> + x instead.
Backtrace:
    ▆
 1. ├─ggplot2::autoplot(out) at [test-prob-gain_curve.R:245:3](vscode-file://vscode-app/Applications/Positron.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html#)
 2. └─yardstick:::autoplot.gain_df(out)
 3.   └─... %+% ggplot2::theme_bw() at [yardstick/R/prob-gain_curve.R:330:3](vscode-file://vscode-app/Applications/Positron.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html#)
 4.     └─ggplot2:::deprecate_soft0("4.0.0", I("<ggplot> %+% x"), I("<ggplot> + x"))