Please add user-session-migration to the ubuntu-desktop packages set

[Marco Trevisan]

As per the recent MIR [1] we are going to replace the usage of the
deprecated session-migration [2] usage with the debian-maintained
user-session-migration [3].

session-migration was already part of the desktop set, so please mark
user-session-migration in the same way so that it will be easy for us to
do manual syncs or potential ubuntu-only changes.

Cheers

[1] https://bugs.launchpad.net/ubuntu/+source/user-session-migration/+bug/2121566
[3] https://launchpad.net/ubuntu/+source/session-migration
[3] https://launchpad.net/ubuntu/+source/user-session-migration