Update social/OG share image to the new Cozystack logo

Drops the refreshed brand mark (icon + "COZYSTACK" wordmark) onto the
existing 1200x630 banner, keeping the background pattern and the
"Free Cloud Platform based on Kubernetes" tagline unchanged.

Follow-up to #551 which updated the in-site logo and favicons.

Signed-off-by: Viktoriia Kvapil <bisunviktoria@gmail.com>

Author: Viktoriia Kvapil