Close all GitHub pull requests automatically.

Change-Id: Ie0849214f578b47d6da2a50205501f42b1572a52
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59692
Reviewed-by: Jeff Bailey <jeffbailey@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed