Track Plasma LTS i18n branches

Right now the (Plasma) LTS i18n branches are not tracked by repo-metadata, only by scripty. As we are trying to reduce the delta between those two data sources, and finally rely on repo-metadata for i18n branches in scripty as well, it would be useful to track the additional branches here too.

We will need new branches anyway when switching to Qt 6.

The proposal is to simply extend the i18n.json file with the additional branches when used. For example the plasma-desktop/i18n.json, currently:

{"trunk": "none", "stable": "none", "stable_kf5": "Plasma/5.19", "trunk_kf5": "master"}

could be changed as

{"trunk": "none", "stable": "none", "stable_lts_kf5": "Plasma/5.18", "stable_kf5": "Plasma/5.19", "trunk_kf5": "master"}

or even removing the default values: {"stable_lts_kf5": "Plasma/5.18", "stable_kf5": "Plasma/5.19", "trunk_kf5": "master"}

This may not have required a task, but it should be taken into account when implementing #1 .