Add a GHA to check PRs on 4.0, blocking them if label is not present (only post release date)

Created by: jhchabran

Adds a GitHub action that will check that after 2022-09-09 the must-be-in-4.0 label is present, or the check will fail otherwise. If we are before that day, the check will always be green.

Later this week, I'll add this check as mandatory.

Test plan

• Run with dday set to 2021-09-09 and no label:
  • 🔴 https://github.com/sourcegraph/sourcegraph/runs/8072123819?check_suite_focus=true
• Run with dday set to 2021-09-09 and with the label:
  • 🟢 https://github.com/sourcegraph/sourcegraph/runs/8072108225?check_suite_focus=true
• Run with dday set to 2022-09-09 and no label:
  • 🟢 https://github.com/sourcegraph/sourcegraph/runs/8072144421?check_suite_focus=true