Update docs for 17.8 upgrades

What does this MR do and why?

Alert users of a possible error that might come up when upgrading to 17.8 related to default work item types created in the work_item_types table.

A user reported the documented problem in #423483 (comment 2385606585) and we had another one through customer support. We are not backporting a fix as we only do bug backports for the current stable release (17.9 at the time of writing). And since 17.8 is a required stop, backporting to 17.9 won't help as everyone has to run the migration in 17.8.

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doc/update/versions/gitlab_17_changes.md

@@ -218,6 +218,33 @@ For more information, see [issue 480328](https://gitlab.com/gitlab-org/gitlab/-/
    gitlab-rake db:migrate:up:ci VERSION=20241028085044
    ```
 
+## Issues to be aware of when upgrading to 17.8
+
+- Migration failures when upgrading to GitLab 17.8.
+
+  When upgrading to 17.8, there is a slight chance of encountering an error. During the migration process, you might see an error message like the one below:
+
+  ```shell
+  ERROR:  duplicate key value violates unique constraint "work_item_types_pkey"
+  DETAIL:  Key (id)=(1) already exists.
+  ```
+
+  The migration producing the error would be `db/post_migrate/20241218223002_fix_work_item_types_id_column_values.rb`.
+
+  This error occurs because in some cases, records in the `work_item_types` table were not created in the database
+  in the same order as they were added to the application.
+
+  To safely resolve this issue, follow these steps:
+
+  1. **Only do this if you got this error when trying to upgrade to 17.8.**
+     Run the following SQL query in your `gitlab_main` database:
+
+      ```sql
+      UPDATE work_item_types set id = (id * 10);
+      ```
+
+  1. Retry running the failed migration. It should now succeed.
+
 ## 17.8.0
 
 - In GitLab 17.8.0, GitLab agent server for Kubernetes (KAS) does not start with the default settings on the GitLab Linux package (Omnibus) and Docker installations.