[12.x] Follow up improvements to #57557

[NickSdot]

Follow up to #57557 as discussed in the comments.

Approach does not change compared to 57557. But in 8.4 we get an advantage because:

• No unnecessary get_object_vars() call; avoids duplicate work.
• Collects desired keys instead of having a call to unset() for each undesired property.
• No unnecessary array_keys() call

Makes it slightly faster (~25%) and a bit cleaner. Nothing massive, but things add up.
Since it's a fresh feature it doesn't hurt to squeeze what we can.

Addtionally:

• Extracted to method to make it overwritable as requested in Exclude property hooks on return of Model::__sleep() #57557 (comment)

[taylorotwell]

Missing doc-blocks.