Set MGet as the claim strategy for serverless (elastic#194694)

In this PR, I'm modifying the `config/serverless.yml` file to contain
`xpack.task_manager.claim_strategy: mget`. We've rolled out the mget
task claimer in phases using the kibana-controller, now that all
projects are using the mget task claiming strategy, we can move the
config here and cleanup all the places in the kibana-controller that set
this flag.

Once this commit rolls out to all serverless projects, I'll be able to
start cleaning up the kibana-controller.

Co-authored-by: Elastic Machine <[email protected]>

config/serverless.yml

@@ -204,6 +204,7 @@ uiSettings:
     labs:dashboard:deferBelowFold: false
 
 # Task Manager
+xpack.task_manager.claim_strategy: mget
 xpack.task_manager.allow_reading_invalid_state: false
 xpack.task_manager.request_timeouts.update_by_query: 60000
 xpack.task_manager.metrics_reset_interval: 120000