Replace public general SLA dashboard URL with metrics dashboard equivalent

Why is this change being made?

This MR makes GitLab's public SLA dashboard the GitLab General SLA Dashboard and updates the reference in the Infrastructure KPIs.

The previous public SLA dashboard Grafana General SLA dashboard will be deprecated after this change is moved.

This is a key step in dogfooding metric dashboards and a goal of the FY21-Q1 OKRs for Ops Section and Infrastructure Department.

Grafana	Metrics Dashboard (Top)	Metrics Dashboard (Bottom)

Note: Screenshots of metrics dashboard are from the runbooks project but there is no difference between those charts and the ones displayed in the new dashboards-gitlab-com repo

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what