Draft PR's timer should start from when it was marked as ready instead of PR creation #1115

Description:
Draft PRs can be set as exempt in the action config. If that is set along with ignore-updates, then once a draft PR is marked as ready, its timer should start from the date it was marked as ready instead of its creation date.

Example config:

      - uses: actions/stale@v8
        with:
          ignore-pr-updates: true
          exempt-draft-pr: true
          days-before-stale: 10
          days-before-close: -1
          days-before-issue-stale: -1
          remove-stale-when-updated: false

Justification:
We are using this action to mark PRs that have been open for a while and need prioritized review. We skip draft PRs because they're not ready for review. A PR could be in draft for a few weeks and then marked as ready. It shouldn't be marked in need of prioritized review until it's been ready for the specified time.

Are you willing to submit a PR?
yes