Bump github.com/prometheus/alertmanager

Bumps [github.com/prometheus/alertmanager](https://github.com/prometheus/alertmanager) from 0.21.1-0.20200911160112-1fdff6b3f939 to 0.25.1.
- [Release notes](https://github.com/prometheus/alertmanager/releases)
- [Changelog](https://github.com/prometheus/alertmanager/blob/v0.25.1/CHANGELOG.md)
- [Commits](https://github.com/prometheus/alertmanager/commits/v0.25.1)

---
updated-dependencies:
- dependency-name: github.com/prometheus/alertmanager
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

go.mod

@@ -9,10 +9,10 @@ require (
 	github.com/evanphx/json-patch/v5 v5.1.0
 	github.com/ghodss/yaml v1.0.0
 	github.com/go-kit/kit v0.10.0
-	github.com/go-openapi/swag v0.19.12
+	github.com/go-openapi/swag v0.22.3
 	github.com/gogo/protobuf v1.3.2
-	github.com/golang/protobuf v1.4.3
-	github.com/google/go-cmp v0.5.4
+	github.com/golang/protobuf v1.5.2
+	github.com/google/go-cmp v0.5.9
 	github.com/hashicorp/go-version v1.2.1
 	github.com/kylelemons/godebug v1.1.0
 	github.com/mitchellh/hashstructure v1.0.1-0.20200508175121-8fdbea448aa6
@@ -21,13 +21,13 @@ require (
 	github.com/prometheus-community/prom-label-proxy v0.2.0
 	github.com/prometheus-operator/prometheus-operator/pkg/apis/monitoring v0.45.0
 	github.com/prometheus-operator/prometheus-operator/pkg/client v0.45.0
-	github.com/prometheus/alertmanager v0.21.1-0.20200911160112-1fdff6b3f939
-	github.com/prometheus/client_golang v1.9.0
-	github.com/prometheus/common v0.15.0
+	github.com/prometheus/alertmanager v0.25.1
+	github.com/prometheus/client_golang v1.14.0
+	github.com/prometheus/common v0.38.0
 	github.com/prometheus/prometheus v1.8.2-0.20210106134434-02e92236a8ba
-	github.com/stretchr/testify v1.6.1
+	github.com/stretchr/testify v1.8.1
 	github.com/thanos-io/thanos v0.17.2
-	golang.org/x/sync v0.0.0-20201207232520-09787c993a3a
+	golang.org/x/sync v0.1.0
 	gopkg.in/alecthomas/kingpin.v2 v2.2.6
 	gopkg.in/yaml.v2 v2.4.0
 	k8s.io/api v0.20.1