plasma kubuntu_stable branches now tracking 5.3

[Harald Sitter]

since upstream branched 5.3 which is the next stable release target
the kubuntu_stable CI branches in our packaging have been adjusted
accordingly.

- kubuntu_unstable (upstream master) was merged into kubuntu_stable
(upstream 5.3)
- CI jobs have been changed to now integrate the 5.3 branch

please note that when packaging 5.3 (pre-)releases you'll want ot
branch off of kubuntu_stable and not kubuntu_unstable.

HS