While GitHub provides the ability to remove old workflow runs (https://github.blog/changelog/2020-07-07-github-actions-ability-to-delete-workflow-runs/), it fails to provide an option to delete all workflow runs for a particular workflow.
Manual deletion can become extremely tedious...
Turns out, this can be achieved using GitHub CLI (https://github.com/cli/cli), read on.
- Idea initially noticed here: refined-github/refined-github#4838