Don't onboard rooms which were created remotely anymore

[mweinelt]

Due to https://matrix.org/blog/2025/07/security-predisclosure/ we should not onboard rooms anymore, which were created on another homeserver, by someone not on the moderation team.

The reason is that the creating account will have infinite permissions and can therefore work around the moderation bot.

Please update https://github.com/NixOS/moderation/blob/main/matrix/adding-rooms.md accordingly, if you agree.