clarify access rights of emeritus members

[h-vetinari]

One of the things that we overlooked in conda-forge/conda-forge.github.io#2501 was to clarify that emeritus members do not retain the "keys to the kingdom" while being emeritus.

Removing the access of emeritus members is self-evident for two reasons:

• Even the most basic security posture requires to not let highly privileged accounts just lying around. Emeritus means "Retired but retaining an honorary title". You'll also have to give back the keys to the office building when you retire from a non-digital job, and the situation is no different here.
• We explicitly place absolutely no hurdles on emeritus members wanting to rejoin core; raising a one-line PR to this repo cannot be considered an undue burden for people wanting to rejoin.

Despite unanimous support for conda-forge/conda-forge.github.io#2501, I assume this PR will need a full vote nevertheless. 🙃

[jaimergp]

Let's write it in a way that is not constrained to Github only.