Replace unsupported HAProxy alerts and dashboards based on mtail metrics

Issues:

• gitlab-com/gl-infra/production-engineering#24866 (closed)
• gitlab-com/gl-infra/production#17237 (closed)
• gitlab-com/gl-infra/production-engineering#23685 (closed)

The new HAProxy 2.8 supports built-in metrics and we will no longer need mtail on HAPorxy nodes once we move to the new HAProxy configurations.

• Revamp frontend.jsonnet service definition
• Refactor frontend dashboards
• Remove unsupported alerts based on mtail metrics