Promoted Member Attribution to a private beta (#15316)

We promote from alpha -> beta so that we don't require the
enableDeveloperExperiments flag, the toggle in the UI is behind the
flag still, so it will only be visible to developers or people using
alpha features.

ghost/core/core/shared/labs.js

@@ -24,14 +24,14 @@ const GA_FEATURES = [
 // NOTE: this allowlist is meant to be used to filter out any unexpected
 //       input for the "labs" setting value
 const BETA_FEATURES = [
-    'activitypub'
+    'activitypub',
+    'memberAttribution'
 ];
 
 const ALPHA_FEATURES = [
     'auditLog',
     'urlCache',
     'beforeAfterCard',
-    'memberAttribution',
     'emailAlerts'
 ];