Update logo to match new purple-styled Mastodon logo

[rugk]

Background

Mastodon has switched its logo and uses a new color: https://blog.joinmastodon.org/2022/06/mastodon-branding-updates/

Proposed solution

The same design patters as the current one (the arrow at the bottom + possibly replacing the letter, maybe an a again which originally stood for auto-redirect, respectively mastodon-auto-remote-follow).

Compare the add-on ID, which is static and thus needs to keep the old name:

mastodon-simplified-federation/src/manifest.json

Line 44 in b7259e0

"id": "mastodon-auto-remote-follow@rugk.github.io",

Alternatives

This could be a chance to come up with a new design in general of the icon.
I would day the arrow needs to be kept (as it is IMGO the best symbol for what this extendion does) and it needs to have some clear connection to the original mastodon logo.

But of course, in general I am very open for any ideas/discussion.

Additional context

Logo assets are here:
https://github.com/rugk/mastodon-simplified-federation/tree/master/assets/logo

Also the screenshots use the arrow and the not finished social media share image, so that would need an update, too: #29

[rugk]

Branding guide: https://joinmastodon.org/de/branding