Rename "xmpp" user service back to "ejabberd"

If we ever add support for others, we can combine them as "xmpp" in
helper methods

spec/jobs/create_ldap_user_job_spec.rb

@@ -44,7 +44,7 @@ RSpec.describe CreateLdapUserJob, type: :job do
 
   it "adds default services for pre-confirmed accounts" do
     allow(ldap_client_mock).to receive(:add) # spy on mock
-    allow(Setting).to receive(:default_services).and_return(["xmpp", "discourse"])
+    Setting.default_services = ["ejabberd", "discourse"]
 
     perform_enqueued_jobs { job_for_preconfirmed_account }
 
@@ -56,7 +56,7 @@ RSpec.describe CreateLdapUserJob, type: :job do
         sn: "halfinney",
         uid: "halfinney",
         mail: "halfinney@example.com",
-        serviceEnabled: ["xmpp", "discourse"],
+        serviceEnabled: ["ejabberd", "discourse"],
         userPassword: "remember-remember-the-5th-of-november"
       }
     )

spec/jobs/xmpp_exchange_contacts_job_spec.rb

@@ -13,7 +13,7 @@ RSpec.describe XmppExchangeContactsJob, type: :job do
   before do
     stub_request(:post, "http://xmpp.example.com/api/add_rosteritem")
       .to_return(status: 200, body: "", headers: {})
-    allow_any_instance_of(User).to receive(:services_enabled).and_return(["xmpp"])
+    allow_any_instance_of(User).to receive(:services_enabled).and_return(["ejabberd"])
   end
 
   it "posts add_rosteritem commands to the ejabberd API" do