.github: move repo lockdown to the v2 configuration

.github: move repo lockdown to the v2 configuration

I was getting prompted by GitHub for new permissions but it turns out
per https://github.com/dessant/repo-lockdown/issues/6:

  Repo Lockdown has been rewritten for GitHub Actions, offering new
  features and better control over your automation presets. The legacy
  GitHub App has been deprecated, and the public instance of the app
  has been shut down.

So this is what I've done. As the issues tab is disabled I've removed
the handling for issues from the new version.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20211004154308.2114870-1-alex.bennee@linaro.org>