Support updating an element in an array with ExecuteUpdate

We currently don't support the following:

```c#
context.Blogs.ExecuteUpdateAsync(s => s.SetProperty(b => b.Tags[0], "foo")
```

The reason is that `b.Tags[0]` translates to the PostgreSQL-specific PgArrayIndexExpression, and the ExecuteUpdate logic for handling setters isn't sufficiently open (see https://github.com/dotnet/efcore/issues/36836).