Port away from KPluginMetaData::serviceTypes
This got deprecated, because the method is for backwards compat and pretty much all consumers are ported away from it & have their metadata files cleaned up.
However in KPackage we need to keep it around, because third parties set this instead of the new property. See https://phabricator.kde.org/T14563.
Silencing the deprecations would more work than reading the config key manually.