[Bug]: Bootstrap class names incorrect in Bootstrap 5 example

[jgbishop]

Bug description

The styling examples documentation page uses the form-control class to style the <select> elements on the page. This works fine for the Bootstrap 4 styling, but in Bootstrap 5, the proper class names for <select> elements should be: form-select (and the size variants, e.g. form-select-sm). See the Bootstrap 5 docs for more.

I'm not familiar with Nunjucks, so I'm not sure what magic would be needed to fix this.

Expected behavior

When selecting the Bootstrap 5 theme, the class names for any <select> elements should be adjusted accordingly.

Steps to reproduce

1. Go to the styling examples page.
2. Change the theme to Bootstrap 5.
3. Note that dropdown in the form-control-sm section isn't styled properly (it is missing the dropdown arrow).

Additional context

N/A