Revert CI approval to first time contributors (or at least discuss it more)

Follow up from: https://github.com/OpenFreeEnergy/openfe/pull/1649#discussion_r2479809724

Some background

The decision to loosen the approval restrictions to only be first time contributors was made a long while ago to a) not block contributions by external parties / turn them off from contributing, b) reduce the pain we had in continually approving CI activation for folks that sat outside of the org, c) avoid us using to add a ton of folks temporarily to the org and then being forced to manage the org members (force 2FA, permissions, etc...).

Why I think it should be reversed

It looks like the decision was made in #1649 rather arbitrarily without really justifying any kind of improvements gained from doing this (maybe I'm missing something, apologies if so), except from just "more security".

Several of the other projects I work on also do "first contributions only", so unless there's an actual advantage, I would suggest reverting to that.