[7.x] Optimize Arr::forget() method

[BSN4]

This PR replaces while:array_shift with foreach:unset just like the other two PRs did #32192 & #32282

[nguyenyou]

https://www.php.net/manual/en/function.current.php#116128
This note on PHP documentation said:

If you do current() after using unset() on foreach statement, you can get FALSE in PHP version 5.2.4 and above.

Don't know if this should be considered...

[BSN4]

didn't saw this not I guess we need array_shift ?

[BSN4]

@nguyenyou tested current and reset both works on php 7.4
reset is guaranteed to be the first item while current is based on the pointer at calltime

[taylorotwell]

Let's hold off on this for now. I don't want to introduce subtle bugs for very little real world performance gain.

[BSN4]

@taylorotwell thanks