[8.x] Update isGuarded to evaluate column names before "->"

[yihyang]

As discussed in #34630, the update in https://github.com/laravel/framework/pull/33777/files removed the ability to update nested attributes using "->" as the comparison of the updating attribute and guarded columns are not done using the key that existed before "->" inside the updating attribute.

This PR is to allow it by restoring the previous code: Use the key that are before "->" of the updating attribute when comparing it with the guarded columns.

[taylorotwell]

I keep saying this is insecure but people keep sending it 🤷‍♂️

[yihyang]

@taylorotwell I just had a look at https://blog.laravel.com/security-release-laravel-61835-7240
I think your preference is to have the users to explicitly set $fillable right? If that's the case would it be better if we make the $fillable wildcard instead? Having a $fillable = ['devices->*'] that allows user to specify which nested JSON attributes that they are allowing.

[yihyang]

Also should we consider remove this section from the documentation?
https://laravel.com/docs/8.x/queries#updating-json-columns