Fix missing backticks in errors

These weren't consistent. In some errors we were using backticks and
others we weren't. This fixes `connected_to` errors to use backticks
around the method name

activerecord/lib/active_record/connection_handling.rb

@@ -135,7 +135,7 @@ def connects_to(database: {}, shards: {})
     #
     # The database kwarg is deprecated and will be removed in 6.2.0 without replacement.
     def connected_to(database: nil, role: nil, shard: nil, prevent_writes: false, &blk)
-      raise NotImplementedError, "connected_to can only be called on ActiveRecord::Base" unless self == Base
+      raise NotImplementedError, "`connected_to` can only be called on ActiveRecord::Base" unless self == Base
 
       if database
         ActiveSupport::Deprecation.warn("The database key in `connected_to` is deprecated. It will be removed in Rails 6.2.0 without replacement.")

activerecord/test/cases/base_test.rb

@@ -1674,7 +1674,7 @@ def test_protected_environments_are_stored_as_an_array_of_string
       Bird.connected_to(role: :reading) { }
     end
 
-    assert_equal "connected_to can only be called on ActiveRecord::Base", error.message
+    assert_equal "`connected_to` can only be called on ActiveRecord::Base", error.message
   end
 
   test "preventing writes applies to all connections on a handler" do