Include 2.4.0p26 in base-versions list for update-test

Author: Checkmk release system

tests/update/base_versions_current_branch.json

@@ -24,5 +24,6 @@
   "2.4.0p22",
   "2.4.0p23",
   "2.4.0p24",
-  "2.4.0p25"
+  "2.4.0p25",
+  "2.4.0p26"
 ]