Update issue templates and labeler for v2.6 release (#post-2.6)
- Add ver: 2.6.x label and keys to advanced-issue-labeler.yml
- Remap master branch label to ver: 2.7.x
- Add v2.6 option to bug report version dropdown
- Update environment template example version to 2.6.0
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>