Adjusted browser support to include Safari 12

no issue

- Safari 12 support was automatically dropped when Safari 14 was released this September. This created quite a step-change in level of JS that is left as native in the build output causing problems for users who haven't upgraded or are stuck on browser versions ~1yr old
- adjusted the browser support to include last 3 versions of Safari, giving another year (most likely based on Apple's historic release schedule) of older browser support

ghost/admin/config/targets.js

@@ -3,7 +3,7 @@
 const browsers = [
     'last 2 Chrome versions',
     'last 2 Firefox versions',
-    'last 2 Safari versions',
+    'last 3 Safari versions',
     'last 2 Edge versions'
 ];