Add annotations to Grafana dashboard about deployment blockers

What does this MR do and why?

• Add annotations to Grafana dashboard about deployment blockers

Instead of manually adding annotations to the panel in the Grafana dashboard "delivery: Auto-Deploy packages information", the new rake task weekly (together with the weekly report) runs and add the corresponding annotations to the deployment blocker issues:

• Use RootCause as annotation's tag
• Link to the issue as text
• Point-in-time annotation if there is no Deploys-blocked label
• If there is at least one Deploys-blocked label, use the longest one and calculate from the created_at time to get the duration

Result

This is how the dashboard panel looks like after running the task (https://dashboards.gitlab.net/goto/D8sagbvIR?orgId=1):

Author Check-list

• Has documentation been updated?
• Add GRAFANA_URL and GRAFANA_API_KEY to CICD variables on Ops. These variables are available in the group https://ops.gitlab.net/gitlab-com/gl-infra/. You can see that in https://ops.gitlab.net/gitlab-com/gl-infra/deployer/-/settings/ci_cd
• Create a feature flag grafana_annotations on Ops https://ops.gitlab.net/gitlab-org/release/tools/-/feature_flags
• Create a follow-up issue to drop the feature flag from our codebase gitlab-com/gl-infra/delivery#19954