scripts: release: set per_page=100 for issue requests

GitHub issue API allows a max of 100 entries per page (up to a default
of 30). Use it for list_issues, makes the script runs a lot faster.

Api documentation:
  https://docs.github.com/en/rest/issues/issues
  https://docs.github.com/en/rest/pulls/pulls



Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>