[devel] New 4k RSA key for apt.ledgersmb.org repository

It had been commented by various people that they have been seeing error messages regarding the apt.ledgesmb.org repository key being too small or too "week" when the repository was being accessed, like the following example comment:

apt update returns W: http://apt.ledgersmb.org/dists/xenial/InRelease: Signature by key 1ED705D54028F70F45E32340D0F530B7F93ECF4F uses weak digest algorithm (SHA1) at the end

I've created a 4K RSA key for the repository and have added it to the configuration there, and updated the home page about it. Will be using both the old and the new key for a bit, but then the old key will be removed from the configuration.