docs: update list of maintainers

🚀 With this change, Yannis Barlas, @yannis, a legend in the Coko world, a great developer and a wonderful person, becomes a PubSweet maintainer. 🎉 I wish you all the best, Yannis, and please don't hesitate to reach out if PubSweet isn't behaving!

Additionally, today is my last day at Coko and as PubSweet maintainer, so I've removed myself from that list there. It was my pleasure to be PubSweet's lead developer and maintainer for the past 5 years! Thank you all!

I also removed Peter, as he is no longer with eLife or otherwise involved with PubSweet. Thank you so much for your contributions, Peter!