[ci] Increase parallelism for defend workflows tests (#169902)

These tests are timing out

https://buildkite.com/elastic/kibana-on-merge/builds/37377
https://buildkite.com/elastic/kibana-on-merge/builds/37374
https://buildkite.com/elastic/kibana-on-merge/builds/37373


@patrykkopycinski @MadameSheema when you get a chance can you help
operations understand how the cypress parallelism balancing works?
There's enough compute prior to this change (although it's close), and
some groups are running quicker. Does it help to split suites up so they
run quicker?

.buildkite/pipelines/es_serverless/verify_es_serverless_image.yml

@@ -102,7 +102,7 @@ steps:
           queue: n2-4-virt
         depends_on: build
         timeout_in_minutes: 60
-        parallelism: 4
+        parallelism: 6
         retry:
           automatic:
             - exit_status: '*'

.buildkite/pipelines/on_merge.yml

@@ -157,7 +157,7 @@ steps:
       queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 60
-    parallelism: 8
+    parallelism: 10
     retry:
       automatic:
         - exit_status: '*'
@@ -169,7 +169,7 @@ steps:
       queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 60
-    parallelism: 4
+    parallelism: 6
     retry:
       automatic:
         - exit_status: '*'

.buildkite/pipelines/pull_request/base.yml

@@ -135,7 +135,7 @@ steps:
       queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 60
-    parallelism: 8
+    parallelism: 10
     retry:
       automatic:
         - exit_status: '*'
@@ -147,7 +147,7 @@ steps:
       queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 60
-    parallelism: 4
+    parallelism: 6
     retry:
       automatic:
         - exit_status: '*'