Element logo in README uses private image so doesn't display

[hughns]

The image source is currently https://github.com/element-hq/product/assets/87339233/7abf477a-5277-47f3-be44-ea44917d8ed7 which won't display if you are not authorised to view the private element-hq/product repository:

[sandhose]

This should be fixed by #17387