Fix query error on Rails 6.1

Rails 6.1 does not allow arbitrary strings in ActiveRecord methods so we
need to wrap this in `Arel.sql`

app/models/postgresql/replication_slot.rb

@@ -27,7 +27,7 @@ module Postgresql
 
       # We force the use of a transaction here so the query always goes to the
       # primary, even when using the EE DB load balancer.
-      sizes = transaction { pluck(lag_function) }
+      sizes = transaction { pluck(Arel.sql(lag_function)) }
       too_great = sizes.compact.count { |size| size >= max }
 
       # If too many replicas are falling behind too much, the availability of a

spec/models/postgresql/replication_slot_spec.rb

@@ -24,6 +24,10 @@ RSpec.describe Postgresql::ReplicationSlot do
       expect(described_class).to receive(:in_use?).and_return(true)
     end
 
+    it 'does not raise an exception' do
+      expect { described_class.lag_too_great? }.not_to raise_error
+    end
+
     it 'returns true when replication lag is too great' do
       expect(described_class)
         .to receive(:pluck)