Fix KCM duplicates in landingpage
In systemsettings, we no longer write a ".desktop" at the end of the KCM ids. This change was made during Plasma 5.24. However, it causes duplicates on some setups. To avoid this, we ignore all of the old .desktop URLs. To ensure that we fill all of the 6 available items, the limit is increased to 12 and we check against a list how many results we have ignored.
In addition to that, we should write a script that deletes all the old .desktop entries. However, this is far more risky than such a change and thus I'd like to avoid backporting it.