[8.x] Extend PostgreSQL updateFrom to support nested joins

[peanutsmcgee]

In reference to #40138

A recent PR (#39151) introduced the updateFrom() method, an alternative way of formatting PostgreSQL update queries that allows the set clause to reference the joined table. It is missing support for nested joins, which the original update() method does support.

[taylorotwell]

I am closing this pull request because it lacks sufficient explanation, tests, or both. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests. Please include it's benefit to end users; the reasons it does not break any existing features; how it makes building web applications easier, etc.

Thanks!