feat(ci): use reusable workflow call for list-open-prs

[wdconinc]

Briefly, what does this PR introduce?

This PR experiments with using a reusable workflow for (in this instance) list-open-prs. By placing this in a dedicate actions repository, it can be more easily reused across our repositories.

What kind of change does this PR introduce?

• [ ] Bug fix (issue #__)
• [x] New feature (issue #__)
• [ ] Documentation update
• [ ] Other: __

Please check if this PR fulfills the following:

• [ ] Tests for the changes have been added
• [ ] Documentation has been added / updated
• [ ] Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.