Upgrade alertmanager v0.32.1

[SungJin1212]

Upgrade the Prometheus alertmanager version to v0.32.1

Key Alertmanager Changes

Protobuf Migration

• clusterpb.FullState migrated from github.com/gogo/protobuf to google.golang.org/protobuf

UI Embed

Alertmanager UI assets (app/dist/) are now embedded via //go:embed app/dist in ui/web.go. The dist/ entry in .gitignore must explicitly allow vendored UI assets via !vendor/**/dist/.

Silence API Model Restructure

The swagger-generated API models now split Silence into separate types:

• • Silence — base model for creating silences (POST), with all fields as required pointers (*string, *strfmt.DateTime)
• • GettableSilence — extends Silence with ID, Status, and UpdatedAt fields (GET responses)
• • PostableSilence — extends Silence with optional ID (for upsert)
• createdBy field is now required when creating a silence via the HTTP API
• Matchers use open_api_models.Matchers ([]*Matcher) instead of amlabels.Matchers
• Silence state constants renamed: SilenceStateActive/Expired → SilenceStatusStateActive/SilenceStatusStateExpired

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags

[SungJin1212]

We need a CI to inject an alertmanager ui assets to the vendor automatically.