Move from `max( id )` to `max( index )` for latest commit statuses (#30076)

This PR replaces the use of `max( id )`, and instead using ``max(
`index` )`` for determining the latest commit status. Building business
logic over an `auto_increment` primary key like `id` is risky and
there’re already plenty of discussions on the Internet.

There‘s no guarantee for `auto_increment` values to be monotonic,
especially upon failures or with a cluster. In the specific case, we met
the problem of commit statuses being outdated when using TiDB as the
database. As [being
documented](https://docs.pingcap.com/tidb/stable/auto-increment),
`auto_increment` values assigned to an `insert` statement will only be
monotonic on a per server (node) basis.

Closes #30074.

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