docs: add deprecation notices for rebase on fork PRs with bot_account (#10817)

The git-based rebase code path for fork PRs with bot account
impersonation will be removed on July 1, 2026. Add caution/note
callouts to affected documentation pages:

- rebase action: recommend switching to the update action
- rebase command: recommend switching to the update command
- merge queue parallel checks: warn about update_bot_account + rebase
- merge strategies: warn about update_method=rebase on fork PRs

Relates to Mergifyio/monorepo#27909 and Mergifyio/monorepo#27959.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kozlek

src/content/docs/commands/rebase.mdx

@@ -19,6 +19,12 @@ import RebaseAdvantage from './_rebase_advantages.mdx';
 
 <RebaseAdvantage />
 
+:::caution
+  Rebasing fork pull requests using `bot_account` impersonation is
+  deprecated and will stop working on **July 1, 2026**. Use the
+  [`update` command](/commands/update) instead for fork PRs.
+:::
+
 ## Syntax
 
 To make use of the `rebase` command, simply comment on the desired pull

src/content/docs/merge-queue/merge-strategies.mdx

@@ -178,6 +178,13 @@ In this mode, `update_method` must be set to `rebase` (the default). If a
 rebase update occurs, the commit SHAs on the PR will change — what fast-forward
 preserves are the SHAs of the PR branch at merge time.
 
+:::caution
+  When using `update_bot_account` with fast-forward inplace mode, fork pull
+  requests will no longer be supported after **July 1, 2026**. See the
+  [update method deprecation note](#combining-merge-and-update-methods) for
+  details and migration options.
+:::
+
 ```dot class="graph"
 strict digraph {
     fontname="sans-serif";
@@ -276,6 +283,12 @@ The `update_method` option controls how Mergify updates PR branches when they
 fall behind the base branch. Combining `merge_method` with `update_method`
 gives you additional control over your history shape.
 
+:::note
+  `update_method: rebase` combined with `update_bot_account` will no longer
+  support fork pull requests after **July 1, 2026**. If your repository
+  receives fork PRs, use `update_method: merge` instead.
+:::
+
 ### Semi-Linear History (Rebase + Merge Commit)
 
 ```yaml

src/content/docs/workflow/actions/rebase.mdx

@@ -17,6 +17,13 @@ up-to-date with the latest changes from the base branch.
   before being merged.
 :::
 
+:::caution
+  The `rebase` action with `bot_account` will no longer support fork pull
+  requests after **July 1, 2026**. If you use this configuration on
+  repositories that receive PRs from forks, switch to the
+  [`update` action](/workflow/actions/update) instead.
+:::
+
 ## Parameters
 
 <ActionOptionsTable def='RebaseActionModel'/>