[github] Close PRs created on the V8 GitHub mirror automatically

As per https://v8.dev/docs/contribute: Code contributions to v8 are
done through Gerrit, not using GitHub pull requests.

Unfortunately not a lot of people seem to read this and end up opening
PRs on the github v8 mirror which then needs to be closed manually:
https://github.com/v8/v8/pulls?q=is%3Apr+is%3Aclosed

Rather than manually closing these PRs, once we add this config, we
can make the google GitHub bot automatically do this. See:
https://opensource.google/docs/github/disable-features/#close

Bug: v8:11301
Change-Id: I2b55f60cb095bd4d3e26e5a096dac2c1c886ba31
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617080Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72032}

.github/mistaken-pull-closer.yml

+# `true` will close all PRs.
+filters:
+  - true
+
+# The message to post to the closed PR.
+commentBody: |
+  Thanks for your contribution!  Unfortunately, we don't use GitHub pull
+  requests to manage code contributions to this repository.  Instead, please
+  see https://v8.dev/docs/contribute which provides full instructions on
+  how to get involved.
+
+# Whether to add a label to the closed PR.
+addLabel: false