feat: change alertmanagerAPIPath to v2

[jsedy7]

What this PR does

This change responds to the recent Alertmanager upgrade, where the deprecation of the api/v1 path was announced in version 0.27.0 Alertmanager-PR2970. From version 0.28.0, Alertmanager plans to drop this path, hence I am making this small change for Mimirtool, which currently uses the deprecated path.

[CHANGE] Alertmanager: Deprecates the v1 API. All v1 API endpoints now respond with a JSON deprecation notice and a status code of 410. All endpoints have a v2 equivalent. The list of endpoints is: #7103

The new Mimir version 2.12.0 also announced a change to the Alertmanager V1 API in the changelog, so it seems appropriate to have this change ready for the next release.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

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[alexweav]

The only APIs that moved to V2 are the ones inside prometheus alertmanager itself - i.e. the ones hosted under the /alertmanager subpath.

Mimir's APIs for managing alertmanager configuration (/api/v1/alerts, different from /alertmanager/api/v1/alerts confusingly) are not deprecated and are still V1. So, /api/v1/alerts is still correct - I think this may cause mimirtool to try to also edit configuration against endpoints that don't exist.

[56quarters]

Given @alexweav's explanation, I'm going to close this PR. Please reopen if you think there's been a mistake!