Port SLA dashboard from Grafana to GitLab

This is a port of the SLA dashboard from: https://dashboards.gitlab.net/d/general-slas/general-slas?orgId=1&from=now-7d&to=now

so it can be used in our GitLab monitoring solution

Here's the dashboard running in the Dogfooding project on ops.gitlab.net

https://ops.gitlab.net/gitlab-com/metrics-dogfooding/-/environments/266/metrics?dashboard=.gitlab%2Fdashboards%2Fsla.yml

The single stat percentile values will be fixed when this is merged: gitlab-org/gitlab!28365 (merged)