update pixelkarte metadata legend

update and unify metadata legends for the following layers
* ch.swisstopo.pixelkarte-pk25.metadata
* ch.swisstopo.pixelkarte-pk50.metadata
* ch.swisstopo.pixelkarte-pk100.metadata
* ch.swisstopo.pixelkarte-pk200.metadata
* ch.swisstopo.pixelkarte-pk500.metadata