chore: update retainer app version && changes

apps/emqx_retainer/src/emqx_retainer.app.src

@@ -2,7 +2,7 @@
 {application, emqx_retainer, [
     {description, "EMQX Retainer"},
     % strict semver, bump manually!
-    {vsn, "5.0.12"},
+    {vsn, "5.0.13"},
     {modules, []},
     {registered, [emqx_retainer_sup]},
     {applications, [kernel, stdlib, emqx, emqx_ctl]},

changes/ce/feat-10782.en.md

@@ -0,0 +1,2 @@
+Added a new `deliver_rate` option to the retainer configuration, which can limit the maximum delivery rate per session in the retainer.
+