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/SupportEraser/plugin.json

@@ -3,6 +3,6 @@
     "author": "Ultimaker B.V.",
     "version": "1.0.1",
     "description": "Creates an eraser mesh to block the printing of support in certain places",
-    "api": "7.6.0",
+    "api": 7,
     "i18n-catalog": "cura"
 }