chore(deps): update github.com/grafana/prometheus-alertmanager digest to 940bde3

[renovate-sh-app]

This PR contains the following updates:

Package	Type	Update	Change
github.com/grafana/prometheus-alertmanager	replace	digest	18275ca → 940bde3

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PR Summary

Medium Risk
Alertmanager is on the critical alerting path; a fork digest bump can change notification/cluster behavior even with no repo code diff, so regression testing in alerting flows is warranted.

Overview
Renovate bumps the github.com/prometheus/alertmanager → github.com/grafana/prometheus-alertmanager replace from digest 18275ca to 5f85680 in the root go.mod / go.sum and apps/historian/go.mod / go.sum. No application source changes—only the pinned fork revision and checksums.

That fork backs Alertmanager types and behavior used across alerting (e.g. cluster re-exports, config/models). Reviewers should treat this as picking up whatever landed on the fork between those commits, not as a local refactor.

^{Reviewed by Cursor Bugbot for commit 63eaaf1. Bugbot is set up for automated code reviews on this repo. Configure here.}