ci: bump Jenkins maxConcurrentTotal to 9

We have added more powerful hosts with more slots(3 instead of 2)
which should be able to accomodate more Nimbus CI jobs.

https://github.com/status-im/infra-ci/issues/85

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile

@@ -40,7 +40,7 @@ pipeline {
       throttleOption: 'category',
       categories: ['nimbus-eth2'],
       maxConcurrentPerNode: 1,
-      maxConcurrentTotal: 6
+      maxConcurrentTotal: 9
     )
     /* Abort old builds for non-main branches. */
     disableConcurrentBuilds(