Ensure methods skipped by AnimationPlayer::seek are not called

[garychia]

Fixes #80640
The original code simply picked the key closest to the current time after the animation was seeked. This might accidentally call a method that had been skipped.

[TokageItLab]

This entire code will be refactored and gone soon, but it will be good for 4.2

[akien-mga]

Thanks!

[golddotasksquestions]

I tested this with the latest dev build (Godot 4.2 dev4) and it $AnimationPlayer.seek(4.0) worked as expected, so did $AnimationPlayer.advance(4.0) Thanks a lot for the fix!

However one thing I noticed was that
$AnimationPlayer.seek(4.0, true)
seems to have the exact same effect as
$AnimationPlayer.seek(4.0, false) or
$AnimationPlayer.seek(4.0)

The documentation says:

If update is true, the animation updates too, otherwise it updates at process time. Events between the current frame and seconds are skipped.

At first I thought $AnimationPlayer.seek(4.0, true) therefore must be the equivalent of $AnimationPlayer.advance(4.0), meaning in my test project I should see "hi" being printed in the output. But that's not the case.
Then I thought maybe the animation is supposed to be visually different (updated), but that's also not the case.

So I don't know if this is a documentation issue which needs more clarification, or if $AnimationPlayer.seek(4.0, true) should actually behave like advance(), in which case there still is a bug.

[YuriSizov]

Cherry-picked for 4.1.3.