bug: lock closed issues and pull requests workflow

[schalkneethling]

[Invalid workflow file: .github/workflows/lock-closed.yml#L8](https://github.com/mdn/content/actions/runs/2736049067/workflow)
error parsing called workflow "mdn/workflows/.github/workflows/lock-closed.yml@main": workflow is not reusable as it is missing a `on.workflow_call` trigger

[cw118]

Hello, I was comparing the idle and lock-closed workflows and I think this bug may be caused by this, since workflow_dispatch (used for manually running a workflow in the "Actions" tab of a repo) should perhaps be workflow_call instead?

I created #72 for this but I am not really an expert in GitHub Actions so I'm not 100% sure.