Follow-on to #26877.
### What changes were proposed in this pull request?
This PR tweaks the stale PR message to [clarify](https://github.com/apache/spark/pull/24457#issuecomment-571393900) the procedure for reopening a PR after it has been marked as stale.
### Why are the changes needed?
This change should clarify the reopening process for contributors.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
N/A
Closes#27114 from nchammas/SPARK-30173-stale-tweaks.
Authored-by: Nicholas Chammas <nicholas.chammas@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
### What changes were proposed in this pull request?
This PR adds [a GitHub workflow to automatically close stale PRs](https://github.com/marketplace/actions/close-stale-issues).
### Why are the changes needed?
This will help cut down the number of open but stale PRs and keep the PR queue manageable.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
I'm not sure how to test this PR without impacting real PRs on the repo.
See: https://github.com/actions/stale/issues/32Closes#26877 from nchammas/SPARK-30173-stale-prs.
Authored-by: Nicholas Chammas <nicholas.chammas@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
