add building field to School preset

see https://github.com/openstreetmap/iD/issues/9341

data/presets/amenity/school.json

@@ -8,7 +8,8 @@
         "grades",
         "religion",
         "denomination",
-        "website"
+        "website",
+        "building_area"
     ],
     "moreFields": [
         "capacity",

data/presets/leisure/sports_centre.json

@@ -3,7 +3,7 @@
     "fields": [
         "name",
         "sport",
-        "building",
+        "building_area",
         "address",
         "fee",
         "opening_hours",