Running `./build/change-version.js v4.0.0-alpha.6 v4.0.0` fixed this,
so the version change script works fine. I'm presuming instead this
change was just omitted from 35f80bb1, and then wouldn't have
been caught by subsequent runs of `change-version`, since it only
ever replaces the exact old version string specified.

This variable is not used anywhere.

Leftover from #18373.

* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

Less on 2 symbols

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.