RFR: 2326: Don't send the ready for sponsor comment to mailing list if the pr is closed

[Zhao Song]

After we deployed [SKARA-2302](https://bugs.openjdk.org/browse/SKARA-2302) and [SKARA-2322](https://bugs.openjdk.org/browse/SKARA-2322), some users complained about receiving numerous "ready for sponsor" emails from closed PRs.
We should prevent the bot from retroactively sending out these emails.

-------------

Commit messages:
 - SKARA-2236

Changes: https://git.openjdk.org/skara/pull/1676/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1676&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2326
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1676.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1676/head:pull/1676

PR: https://git.openjdk.org/skara/pull/1676