Remove outdated auto-merge limitation for stacked pull requests

Auto-merge works with stacked pull requests since they became generally
available: https://github.blog/changelog/2026-10-06-stacked-pull-requests-generally-available/

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AugnFjhMJiq9sxScLYogcu
Co-authored-by: nicholas.dean <nicholas.dean@outset.ai>

Author: claude

content/pull-requests/how-tos/merge-and-close-pull-requests/merging-stacked-pull-requests.md

@@ -26,7 +26,6 @@ If the stack is not linear, for example, after changes were pushed to a lower br
 
 > [!NOTE]
 > * If you merge via the API and want to use stacked pull requests, you'll need use the asynchronous merge API for stacks. See [AUTOTITLE](/rest/pulls/pulls?apiVersion=2026-03-10#merge-a-pull-request-asynchronously).
-> * Auto-merge is not supported for stacked pull requests.
 
 ## Merging using a merge queue