Update guide for 9.3

See merge request !12038

 ==
https://gitlab.com/gitlab-org/gitlab-ee/commit/30304bd2ab3c87d6304a3d4c4
bd71ab7ffd2794b from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2265

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33223

[ci skip]

This reverts merge request !12195

This adds an ID-less table containing one row per file, per merge request
diff. It has a column for each attribute on Gitlab::Git::Diff that is serialised
currently, with the advantage that we can easily query the attributes of this
new table.

It does not migrate existing data, so we have fallback code when the legacy
st_diffs column is present instead. For a merge request diff to be valid, it
should have at most one of:

* Rows in this new table, with the correct merge_request_diff_id.
* A non-NULL st_diffs column.

It may have neither, if the diff is empty.

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