Update branches to test against with new LTS: master, 3.24.x and 3.21.x

Ticket: ENT-11734
Changelog: none

Author: craigcomstock

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on:
   # run this workflow on pull_request activity
   # this includes opening and pushing more commits
   pull_request:
-    branches: [ master, 3.21.x, 3.18.x ]
+    branches: [ master, 3.24.x, 3.21.x ]
 
 jobs:
   unit_tests: