OBSDOCS-2772 Update Alertmanager upstream version link for OCP 4.21 release

[eromanova97]

Version(s): 4.21

Issue: https://issues.redhat.com/browse/OBSDOCS-2772

Link to docs preview:

• https://104929--ocpdocs-pr.netlify.app/openshift-monitoring/latest/configuring-core-platform-monitoring/configuring-alerts-and-notifications#configuring-alert-routing-default-platform-alerts_configuring-alerts-and-notifications
• https://104929--ocpdocs-pr.netlify.app/openshift-monitoring/latest/configuring-user-workload-monitoring/configuring-alerts-and-notifications-uwm#configuring-alert-routing-user-defined-alerts-secret_configuring-alerts-and-notifications-uwm

QE review: n/a, version approved here: #104870

Additional information:

[ocpdocs-previewbot]

🤖 Mon Jan 19 11:29:59 - Prow CI generated the docs preview:
https://104929--ocpdocs-pr.netlify.app
Complete list of updated preview URLs: artifacts/updated_preview_urls.txt

[eromanova97]

Hello! Yes, it was an issue in the past when we linked to the different version instead of the version we supported for that particular release. However, I am trying to make this a little bit more effective by creating the attribute for it 👍

[max-cx]

Peer review done.
Good luck with the improvement!

[openshift-ci]

@eromanova97: all tests passed!

Full PR test history. Your PR dashboard.

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[eromanova97]

/cherrypick monitoring-docs-4.21

[openshift-cherrypick-robot]

@eromanova97: new pull request created: #104942

Details

In response to this:

/cherrypick monitoring-docs-4.21

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