Fix dropdown navigation z-index to cover other content underneath

[rioksane]

Problem: If a page has youtube embed on top, it shows through dropdown navigation on top

Example: https://beta.plone.org/news-and-events/plone-youtube-channel

Solution: make main navigation z-inder higher.

[rioksane]

Thanks Brian D. Here's your culprit for the youtube videos https://github.com/plone/volto/blob/54dfaff35d08b50e946f25f37a6400103940e971/theme/themes/pastanaga/extras/blocks.less#L167-L176 vs https://github.com/collective/volto-dropdownmenu/blob/master/src/components/dropdownmenu.css#L25-#L29 = both z-index: 2