Temporarily allow issue-bot to fail (backport to 18-11-stable)

What does this MR do and why?

Backport of the fix from master (commit de1a30ae) to 18-11-stable.

The issue-bot job was failing on protected branches (stable branches, master, tags) with:

unable to create GitLab client(s): please configure "ISSUE_BOT_API_TOKEN" or "ISSUE_BOT_API_TOKEN_FILE" environment variable

This adds allow_failure: true to the issue-bot job on the 18-11-stable branch to prevent pipeline failures while the token configuration is being fixed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

Related issues

Closes #2302 (closed)