feat: fetch all repo PRs instead of just open PRs

[tranhl]

Description

Fixes #13

Decided to fetch all PRs instead of open ones because:

• The action can fail on closed PRs as well, not just merged PRs.
• It's not possible to request merged PRs via the pulls REST API. We'd need to call Check if a pull request has been merged instead, which is more trouble than it's worth.

Stack

• main
  • feat: fetch all repo PRs instead of just open PRs #41 👈