fix: close the nav when it loses focus, e.g keyboard user tabs out of it

[shsteimer]

Please always provide the GitHub issue(s) your PR is for, as well as test URLs where your change can be observed (before and after):

Fix https://github.com/adobe/aem-block-collection/issues/8

https://header-nav-a11y-fix--franklin-playground--shsteimer.hlx.live/

[aem-code-sync[bot]]

Page	Scores	Audits	Google
/