Support Leap to Leap and Leap Micro to Leap Micro upgrade to a new version

[lkocman]

Hello

for the convinience of any repository changes I'd like to make sure that

Leap 15.6 -> Leap 16.0 and later 16.0 -> 16.1 is supported. I expect people to migrate from 15.4 (Dr T) to 16.0 and similar. https://en.opensuse.org/SDB:System_upgrade

Same for Leap Micro 6.0 ->6.1 should be supported too. This is documented here https://en.opensuse.org/SDB:System_upgrade_to_LeapMicro_6.0

This is not necessarily a migration but rather upgrade.

In case of Leap Micro it has to be executed in transactional-update shell, we could have a little check for writeability to / and exit with error code saying please run this in transactional-update shell.

Steps are the same for both.
install openSUSE-repos-$distro from the migration target, zypper ref -s (important to generate all repo files), zypper --releasever $VER dup. reboot