Popover example shows above the navbar

1. https://twbs-bootstrap.netlify.app/docs/5.0/components/popovers/#example
2. Open popover
3. Scroll downwards

v4 isn't affected AFAICT.

EDIT:

Changing .bd-subnav's z-index to 1070 fixes the issue for me. But I guess it should be bigger than the highest core z-index so maybe we should go with 1080?

$zindex-dropdown:                   1000;
$zindex-sticky:                     1020;
$zindex-fixed:                      1030;
$zindex-modal-backdrop:             1040;
$zindex-modal:                      1050;
$zindex-popover:                    1060;
$zindex-tooltip:                    1070;