Update icons declaration

[alizedebray]

Angular 18 offers to migrate to a new build system.
However, when doing so the build results in an error with the icons.

To avoid this error, the icons should be declared as:

// RIGHT
$_accordion-button-icon: icons.get-colored-svg-url(2113, $accordion-button-color);
$accordion-button-icon: url('#{$_accordion-button-icon}');

// WRONG
$accordion-button-icon: url('#{icons.get-colored-svg-url(2113, $accordion-button-color)}');