Update "late scheduling" response to advise on Monday scheduling

[Squirrelginger]

Chesster's current response to an erroneous attempt to schedule a game on the first Monday is ":x: @user Hi! Sorry, that time you posted is not an acceptable time. We need all games to end by 12:00 GMT on Monday, and we believe if you start then, you won't be done then! Please try and find a better time, and if you cannot, please contact the moderators." But it more often appears when a user is trying to schedule a game for the first Monday of the week, and causes confusion.
Can something like "If you're trying to schedule on the first Monday of the week, use the date: e.g. 2021-09-20 13:00UTC" be added in?

[mathgeek]

#231 is open to handle actually scheduling on the first Monday if viable - we might just go ahead and fix that instead