FEATURE: allow S3 ACLs to be disabled (#21769)

AWS recommends running buckets without ACLs, and to use resource policies to manage access control instead.
This is not a bad idea, because S3 ACLs are whack, and while resource policies are also whack, they're a more constrained form of whack.
Further, some compliance regimes get antsy if you don't go with the vendor's recommended settings, and arguing that you need to enable ACLs on a bucket just to store images in there is more hassle than it's worth.
The new site setting (s3_use_acls) cannot be disabled when secure
uploads is enabled -- the latter relies on private ACLs for security
at this point in time. We may want to reexamine this in future.

config/site_settings.yml

@@ -1450,6 +1450,8 @@ files:
     regex: '^https?:\/\/.+[^\/]$'
   s3_configure_tombstone_policy:
     default: true
+  s3_use_acls:
+    default: true
   enable_s3_inventory:
     default: false
   s3_configure_inventory_policy: