Added v2.6.0-RC2 to semaphore builds (#1832)

Author: pranavrth

.semaphore/semaphore.yml

@@ -8,7 +8,7 @@ execution_time_limit:
 global_job_config:
   env_vars:
     - name: LIBRDKAFKA_VERSION
-      value: v2.5.3
+      value: v2.6.0-RC2
   prologue:
     commands:
       - checkout