Now unused API field only took integers originally.

We want to keep it this way so people can still make plugins work for older versions of Cura, like 3.6, where the 'api' field with just a single major version, instead of the 'supported_sdk_versions', which can be either a version-string like '7.6.0' or a list of version strings.

plugins/UltimakerMachineActions/plugin.json

@@ -3,6 +3,6 @@
     "author": "Ultimaker B.V.",
     "version": "1.0.1",
     "description": "Provides machine actions for Ultimaker machines (such as bed leveling wizard, selecting upgrades, etc.).",
-    "api": "7.6.0",
+    "api": 7,
     "i18n-catalog": "cura"
 }