Adopt the improved onion logo

The current Tor onion logo (which is also present as the "o" in the full Tor logo, see the styleguide for reference) includes some very thin strokes and layered colors that don't hold up well in different applications. Due to this, @nicob used a simplified version for the cap developed for Defcon 2024 instead (see https://gitlab.torproject.org/tpo/operations/fundraising/-/issues/157).

It would be good to formalize the simplified onion and deploy it wherever the onion logo occurs (e.g. in favicons and the full Tor logo). However as a brand change, this will require approval from the Comms team and senior leadership first. Before we get to that point, we need to:

• Determine whether or not changing the Tor logo would require updating our trademark (and figure out if this is a blocker or not)
• Test the new version against a range of backgrounds, and in a variety of sizes (both the onion logo in isolation and the full Tor logo)
• Prepare a pitch for Comms to adopt the updated logo
• Create a list of all instances of the original logos which will need updated