charm-update-libs fails on repositories with issues disabled

There are some repositories for which issues are tracked outside of GitHub (e.g., private repositories). These issues could still benefit from the `charm-update-libs.yml` workflow, but fail to run it because the workflow exits when it cannot find issues.

I can think of a couple solutions:

1. Add option to disable issues step for these repositories, and expect these repositories to rely on PR library check
2. If `gh issue list --limit 1` command yields `the '/' repository has disabled issues`, then create a [warning](https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-a-warning-message) and skip the step

Start with charm-update-libs.yml and reproduce the failure by running gh issue list --limit 1 on a repository with issues disabled. Decide which of the two proposed behaviors the workflow should use, then verify that the workflow completes while retaining the PR library check and appropriately handles the unavailable issues step.

Written by the indexing model from the issue text.