Set serverless and test setting for task manager state validation (#163743)

Part of https://github.com/elastic/kibana/issues/155764.

In this PR, I'm setting the
`xpack.task_manager.allow_reading_invalid_state` serverless setting to
`false` so Kibana doesn't allow reading invalid state when loading
tasks.

I'm also doing the same for the functional tests to ensure valid task
state is always read.

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

config/serverless.yml

@@ -106,3 +106,6 @@ xpack.alerting.rules.run.ruleTypeOverrides:
     timeout: 1m
 xpack.alerting.rules.minimumScheduleInterval.enforce: true
 xpack.actions.run.maxAttempts: 10
+
+# Task Manager
+xpack.task_manager.allow_reading_invalid_state: false

x-pack/test/alerting_api_integration/common/config.ts

@@ -333,6 +333,7 @@ export function createTestConfig(name: string, options: CreateTestConfigOptions)
               ]
             : []),
           '--notifications.connectors.default.email=notification-email',
+          '--xpack.task_manager.allow_reading_invalid_state=false',
         ],
       },
     };