Somewhat retire the core team

[rylev]

With rust-lang/rfcs#3392 merged, the leadership council has taken over for the core team. Ideally, we would just move the core team to the alumni section, but there are some legacy infrastructure concerns and questions that we need to figure out before retiring core (including but not limited to where should core email addresses route to).

Until we get all of that straightened out, we'll move all remaining core team members who are not on the council to alumni, and we'll remove the permissions that can already be removed.

A big thank you to @badboy for their service on the core team!!