Bug issue template suggests deprecated DEBUG=true instead of GLAB_DEBUG=true

Summary

The bug issue template .gitlab/issue_templates/Bug.md still instructs reporters to run debug mode via the deprecated variable:

Run the command in debug mode (like DEBUG=true glab mr list) and attach any useful output

Following it, glab prints:

DEPRECATION WARNING: The environment variable DEBUG has been deprecated and will be removed in future releases. Use GLAB_DEBUG instead.

Proposal

Update the checklist line to GLAB_DEBUG=true glab mr list.

Related: #7999 (GLAB_ prefix deprecation feedback).