Debian package repository updates

[Doug Torrance]

Debian users:

There have been some updates to the instructions for downloading Macaulay2 Debian packages.  They are now hosted on the main Macaulay2 website instead of people.debian.org, and the repository signing key has been updated.

At some point between now and the release of Macaulay2 1.25.11 in November, you should:

* Remove any references to the old M2 repo at people.debian.org from your /etc/apt/sources.list file.

* Following the instructions from the M2 wiki [1] to set up the new repository and signing key.

Please let me know if you run into any issues!

[1] https://github.com/Macaulay2/M2/wiki/Installing-Macaulay2-in-Linux#debian