Remove 15.9 sanitize todo note

Removes note about making sure this migration runs. The reason is that
if we do todo sanitization, we should rather sanitize all existing
todos because there are probably some other edge cases when a todo is
not deleted. Or even better we might be able to avoid doing this todo
sanitization at all.

Either way, there is no reason to run this sanitization as it will be
superseded by different solution (or more general migration).

doc/update/index.md

@@ -267,7 +267,6 @@ and [Helm Chart deployments](https://docs.gitlab.com/charts/). They come with ap
 ### 15.9.0
 
 - There is a [database migration bug in GitLab 15.9.x](#user-profile-data-loss-bug-in-159x) that can cause data to be lost from the user profile fields. This bug affects all currently available 15.9.x releases. Until a bug fix is released, you should upgrade to 15.6.x, 15.7.x, or 15.8.x first.
-- This version removes `SanitizeConfidentialTodos` background migration [added](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/87908/diffs) in 15.6, which removed any user inaccessible to-do items. Make sure that this migration is finished before upgrading to 15.9.
 - As part of the [CI Partitioning effort](../architecture/blueprints/ci_data_decay/pipeline_partitioning.md), a [new Foreign Key](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/107547) was added to `ci_builds_needs`. On GitLab instances with large CI tables, adding this constraint can take longer than usual. Make sure that this migration is finished before upgrading to 15.9.
 - Praefect's metadata verifier's [invalid metadata deletion behavior](../administration/gitaly/praefect.md#enable-deletions) is now enabled by default.