Replace SLA dashboard on grafana with link to GitLab's metrics dashboard

As part of the Engineering Ops sub-department, one of our FY21-Q1 OKRs was to identify and replace an existing Grafana dashboard.

• We identified the general SLA dashboard as being the lowest impact dashboard to replace
• We cloned the general SLA dashboard into a public facing GitLab metrics dashboard
• We replaced the public general SLA dashboard URL on the infrastructure KPI page to reference the GitLab metrics dashboard's URL as a reference for the KPI metric (www-gitlab-com!47689 (merged))

This MR completes the last step by "deprecating" the general SLA dashboard in Grafana by removing the existing charts and adding a link to the new GitLab metrics dashboard URL.

Before	After

Snapshot (only available for 24 hours from generation): https://dashboards.gitlab.net/dashboard/snapshot/GXDrLb3torMeFtTYOOP7B0qK7VLZyc1S?orgId=1

Approvals

Replace Grafana General SLA Dashboard with link to Gitlab General SLA Dashboard.

• VP of Infrastructure @sloyd
• Senior Director of Development @clefelhocz1
• Director of Infrastructure, Reliability @brentnewton