Warn API key users and bump version to 2.1.5

lib/onboarding.js

@@ -159,6 +159,11 @@ async function runOnboarding() {
       ]
     );
 
+    if (authMode === 'apiKeys') {
+      console.log('Warning: when using API keys, the spreadsheet must be publicly accessible.');
+      console.log('');
+    }
+
     const credentialCount = await askPositiveInteger(
       ask,
       `How many ${authMode === 'apiKeys' ? 'API keys' : 'service accounts'} do you want to add? `