Fix test of index names (now limited to 62 bytes by rails)

See rails/rails@3682aa1

So the shortening of index names that was added to rails itself
was almost immediately changed to use an 'idx_' prefix instead of an 'ix_'
prefix.

See rails/rails@59f2b06

However, now that rails does shortening. It is an opportunity to
simplify this library and just rely on the rails index name shortening.

spec/active_record/connection_adapters/oracle_enhanced/schema_statements_spec.rb

@@ -330,10 +330,10 @@ class ::TestEmployee < ActiveRecord::Base; end
     it "should raise error if too large index name cannot be shortened" do
       if @oracle12cr2_or_higher
         expect(@conn.index_name("test_employees", column: ["first_name", "middle_name", "last_name"])).to eq(
-          ("index_test_employees_on_first_name_and_middle_name_and_last_name"))
+          ("idx_on_first_name_middle_name_last_name_ee1d3958bc"))
       else
         expect(@conn.index_name("test_employees", column: ["first_name", "middle_name", "last_name"])).to eq(
-          "i" + OpenSSL::Digest::SHA1.hexdigest("index_test_employees_on_first_name_and_middle_name_and_last_name")[0, 29]
+          "i" + OpenSSL::Digest::SHA1.hexdigest("idx_on_first_name_middle_name_last_name_ee1d3958bc")[0, 29]
         )
       end
     end