Fixed start/end dates don't make really make sense

All three products are continously updated, the terrain ones in a six
year cycle the orthofoto faster. Making the start and end dates quite
meaningless outside of a rough indication from when the data is.

Maybe it would be a better idea to allow specification of an
"update_cycle" value, which would allow the user at least to determine
the maximum age of the imagery they are using.

Author: simonpoole

sources/europe/ch/swisstopo_swissalti_hillshade.geojson

@@ -11,8 +11,8 @@
         "country_code": "CH",
         "best": false,
         "icon": "https://upload.wikimedia.org/wikipedia/commons/b/b4/Coat_of_Arms_of_Switzerland_%28Pantone%29.svg",
-        "start_date": "2012",
-        "end_date": "2019",
+        "start_date": "2018",
+        "end_date": "2024",
         "category": "elevation",
         "max_zoom": 18,
         "attribution": {

sources/europe/ch/swisstopo_swissimage.geojson

@@ -12,7 +12,6 @@
         "best": true,
         "icon": "https://upload.wikimedia.org/wikipedia/commons/b/b4/Coat_of_Arms_of_Switzerland_%28Pantone%29.svg",
         "start_date": "2017",
-        "end_date": "2020",
         "category": "photo",
         "available_projections": [
             "EPSG:2056",

sources/europe/ch/swisstopo_swisssurface_hillshade.geojson

@@ -11,8 +11,8 @@
         "country_code": "CH",
         "best": false,
         "icon": "https://upload.wikimedia.org/wikipedia/commons/b/b4/Coat_of_Arms_of_Switzerland_%28Pantone%29.svg",
-        "start_date": "2017",
-        "end_date": "2019",
+        "start_date": "2024",
+        "end_date": "2030",
         "max_zoom": 18,
         "category": "elevation",
         "attribution": {