Update CODEOWNERS (#7490)

This commit updates CODEOWNERS to add
@grafana/mimir-ruler-and-alertmanager-maintainers as codeowners
for vendor/github.com/prometheus/alertmanager. This will give
the Alerting team permissions to approve and merge updates to
Alertmanager without having to be in the larger Mimir maintainers
group.

CODEOWNERS

@@ -26,3 +26,4 @@ CHANGELOG.md
 go.mod @grafana/mimir-maintainers @grafanabot
 go.sum @grafana/mimir-maintainers @grafanabot
 /vendor/ @grafana/mimir-maintainers @grafanabot
+/vendor/github.com/prometheus/alertmanager @grafana/mimir-ruler-and-alertmanager-maintainers @grafana/mimir-maintainers @grafanabot