Simplify regexp for splitting recipients

This fixes the warning: character class has duplicated range

With this change we're also accepting a single carriage return
as a separator.

app/models/project_services/pipelines_email_service.rb

@@ -100,6 +100,6 @@ class PipelinesEmailService < Service
   end
 
   def retrieve_recipients(data)
-    recipients.to_s.split(/[,(?:\r?\n) ]+/).reject(&:empty?)
+    recipients.to_s.split(/[,\r\n ]+/).reject(&:empty?)
   end
 end

spec/models/project_services/pipelines_email_service_spec.rb

@@ -305,8 +305,8 @@ describe PipelinesEmailService, :mailer do
     end
 
     context 'with recipients list separating with newlines' do
-      let(:recipients) { "\ntest@gitlab.com,  \r\nexample@gitlab.com" }
-      let(:receivers) { %w[test@gitlab.com example@gitlab.com] }
+      let(:recipients) { "\ntest@gitlab.com,  \r\nexample@gitlab.com\rother@gitlab.com" }
+      let(:receivers) { %w[test@gitlab.com example@gitlab.com other@gitlab.com] }
 
       context 'with failed pipeline' do
         before do