[8.8] [ResponseOps] Enable maintenance windows (#156033) (#156069)

# Backport

This will backport the following commits from `main` to `8.8`:
- [[ResponseOps] Enable maintenance windows
(#156033)](#156033)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Alexi
Doak","email":"[email protected]"},"sourceCommit":{"committedDate":"2023-04-27T17:45:06Z","message":"[ResponseOps]
Enable maintenance windows (#156033)\n\n## Summary\r\n\r\nEnables
maintenance windows by setting the feature flag to
`true`","sha":"ef99b8505d9b1385f14cf551b3c24acfadca6355","branchLabelMapping":{"^v8.9.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","Team:ResponseOps","v8.8.0","v8.9.0"],"number":156033,"url":"https://github.com/elastic/kibana/pull/156033","mergeCommit":{"message":"[ResponseOps]
Enable maintenance windows (#156033)\n\n## Summary\r\n\r\nEnables
maintenance windows by setting the feature flag to
`true`","sha":"ef99b8505d9b1385f14cf551b3c24acfadca6355"}},"sourceBranch":"main","suggestedTargetBranches":["8.8"],"targetPullRequestStates":[{"branch":"8.8","label":"v8.8.0","labelRegex":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"main","label":"v8.9.0","labelRegex":"^v8.9.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/156033","number":156033,"mergeCommit":{"message":"[ResponseOps]
Enable maintenance windows (#156033)\n\n## Summary\r\n\r\nEnables
maintenance windows by setting the feature flag to
`true`","sha":"ef99b8505d9b1385f14cf551b3c24acfadca6355"}}]}]
BACKPORT-->

Co-authored-by: Alexi Doak <[email protected]>

x-pack/plugins/alerting/common/index.ts

@@ -66,4 +66,4 @@ export const INTERNAL_ALERTING_API_GET_ACTIVE_MAINTENANCE_WINDOWS_PATH =
 
 export const ALERTS_FEATURE_ID = 'alerts';
 export const MONITORING_HISTORY_LIMIT = 200;
-export const ENABLE_MAINTENANCE_WINDOWS = false;
+export const ENABLE_MAINTENANCE_WINDOWS = true;