Update gitlab-org/build/omnibus-mirror/alertmanager from 0.20.0 to 0.21.0

The following dependencies have been updated by dependencies.io:

• gitlab-org/build/omnibus-mirror/alertmanager from "0.20.0" to "0.21.0"

Changelog

This release removes the HipChat integration as it is discontinued by Atlassian on June 30th 2020.

- [CHANGE] [HipChat] Remove HipChat integration as it is end-of-life. #2282
- [CHANGE] [amtool] Remove default assignment of environment variables. #2161
- [CHANGE] [PagerDuty] Enforce 512KB event size limit. #2225
- [ENHANCEMENT] [amtool] Add cluster command to show cluster and peer statuses. #2256
- [ENHANCEMENT] Add redirection from / to the routes prefix when it isn't empty. #2235
- [ENHANCEMENT] [Webhook] Add max_alerts option to limit the number of alerts included in the payload. #2274
- [ENHANCEMENT] Improve logs for API v2, notifications and clustering. #2177 #2188 #2260 #2261 #2273
- [BUGFIX] Fix child routes not inheriting their parent route's grouping when group_by: [...]. #2154
- [BUGFIX] [UI] Fix the receiver selector in the Alerts page when the receiver name contains regular expression metacharacters such as +. #2090
- [BUGFIX] Fix error message about start and end time validation. #2173
- [BUGFIX] Fix a potential race condition in dispatcher. #2208
- [BUGFIX] [API v2] Return an empty array of peers when the clustering is disabled. #2203
- [BUGFIX] Fix the registration of alertmanager_dispatcher_aggregation_groups and alertmanager_dispatcher_alert_processing_duration_seconds metrics. #2200
- [BUGFIX] Always retry notifications with back-off. #2290