Bump github.com/prometheus/alertmanager from 0.21.0 to 0.25.1

Bumps [github.com/prometheus/alertmanager](https://github.com/prometheus/alertmanager) from 0.21.0 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/compare/v0.21.0...v0.25.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -5,7 +5,7 @@ go 1.13
 require (
 	github.com/fluffle/goirc v1.1.1
 	github.com/gorilla/mux v1.8.0
-	github.com/prometheus/alertmanager v0.21.0
-	github.com/prometheus/client_golang v1.11.1
+	github.com/prometheus/alertmanager v0.25.1
+	github.com/prometheus/client_golang v1.14.0
 	gopkg.in/yaml.v2 v2.4.0
 )