CI: Remove master from matrix due to Redmine 5 incompatibilities.

We currently do not support Redmine version 5.

Author: Joachim Mathes

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         # We test against the latest Redmine stable and the master versions
-        redmine: ['4.1-stable', 'master']
+        redmine: ['4.2-stable']
 
     steps:
     # We checkout Redmine itself, since plugins can only be tested inside a running Redmine