logo: change SVG file used for the logo

[ upstream commit 8a02976 ]

Replace the SVG for Cilium's logo by a version picked up from [0]. The
latter has a few differences:

- Wider space between logo (hexagons) and text
- Proportionally bigger text
- Darker text
- No white borders inside some hexagons (SVG artifacts)

It also corresponds to the logo found on https://cilium.io (although
that one is a .png).

The logo is used in the top-left corner of the documentation, and at the
top of the rendered README.rst on GitHub.

[0]: https://github.com/cilium/sphinx_rtd_theme/blob/master/docs/demo/static/cilium-logo.svg

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Joe Stringer <joe@cilium.io>