Use model.arel_table in get_all for Rails main compatibility

Rails main (rails/rails#57021) makes Arel::Table.new keyword-only, so the
positional Arel::Table.new(table_name.to_sym) calls in get_all raise
ArgumentError: wrong number of arguments (given 1, expected 0). Because the
memoizer middleware calls preload_all on every request, this breaks every
request on apps tracking Rails main.

ActiveRecord::Base.arel_table returns the same table and is stable across all
supported Rails versions (5.2-8.0 and main), so it avoids the constructor
signature entirely.

Author: kylekeesling

lib/flipper/adapters/active_record.rb

@@ -106,8 +106,8 @@ def get_multi(features)
       def get_all(**kwargs)
         with_connection(@feature_class) do |connection|
           # query the gates from the db in a single query
-          features = ::Arel::Table.new(@feature_class.table_name.to_sym)
-          gates = ::Arel::Table.new(@gate_class.table_name.to_sym)
+          features = @feature_class.arel_table
+          gates = @gate_class.arel_table
           rows_query = features.join(gates, ::Arel::Nodes::OuterJoin)
             .on(features[:key].eq(gates[:feature_key]))
             .project(features[:key].as('feature_key'), gates[:key], gates[:value])