Update connection assist's iconography

We designed new iconography for connection assist last year, but never handed it over for implementation.

I had planned on waiting until the next major revision of torconnect to ship the new icons (see #41621 (closed), and this WIP Figma file), however since the new torconnect icon will be making an appearance in the browser chrome (#41608 (closed)) and connection settings (#41618 (closed)) sooner than planned, we should ship these at the same time in Tor Browser 12.5 for consistency.