[CMake] CMake APT Repository Signing Key

[Kyle Edwards]

All,

The new 2020 signing key for the APT repository
(https://apt.kitware.com/) has been generated for 2020, with
fingerprint A8E5EF3A02600268. The 2019 key, with fingerprint
C1F34CDD40CD72DA, will remain in effect until the beginning of 2020, at
which point the 2020 key will go into effect. The  The 2018 key, with
fingerprint 461FD00B2DB57A2D, has been retired, and will see no further
use.

As a reminder, we recommend that you install the kitware-archive-
keyring package from the APT repository, which will ensure that your
keyring always stays up to date as we rotate our keys. The kitware-
archive-keyring package has been updated to reflect these changes. The
new version is 2019.07.03.

Kyle