[5.8] Use the getDirty method on insert instead of getAttributes

[kirba]

Same as #25559, except this time targeting new version as you said you wouldn't change something like this on a point release.

Below is a comment from previous PR:
In #25349 performInsert() was changed to use getAttributes() instead of accessing $attributes property directly, thus having more similar behavior to performUpdate().

I think it is better to use getDirty() in performInsert() as well.

Currently if we override getAttributes() method and return additional property the insert is going to fail, while update can pass if we override originalIsEquivalent() and return true for that additional property.

[taylorotwell]

I see no point in changing this honestly. It's been this way for years without problems.

[kirba]

@taylorotwell it has been 17 days like this not for years. $attributes property was accessed directly and overriding getAttributes() method didn't cause a problem with performInsert(). Now if we override getAttbibutes() and return some transient attributes there as well we will have a problem with insert. Before 17 days we didn't have this problem.