Bump mail_room to v0.8.1 to fix thread cleanup issue

Closes #20273

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.13.0 (unreleased)
   - Truncate long labels with ellipsis in labels page
+  - Bump mail_room to v0.8.1 to fix thread cleanup issue
   - Update runner version only when updating contacted_at
   - Add link from system note to compare with previous version
   - Improve issue load time performance by avoiding ORDER BY in find_by call

Gemfile.lock

@@ -399,7 +399,7 @@ GEM
       systemu (~> 2.6.2)
     mail (2.6.4)
       mime-types (>= 1.16, < 4)
-    mail_room (0.8.0)
+    mail_room (0.8.1)
     method_source (0.8.2)
     mime-types (2.99.3)
     mimemagic (0.3.0)