Use optimized whereIn() on eager loading for all integer keys

[rimace]

With version 5.7.14 there where some performance optimizations for the whereIn method for relations (noted as "Improved eager loading performance" in the release notes).
This brought a breakage for existing users and this breakage was fixed by workarounds with #26622 and further with #26688.

Currently this workaround uses the optimization and stays compatible with existing apps, by doing the following checking (in Illuminate\Database\Eloquent\Relations\Relation line 320):

protected function whereInMethod(Model $model, $key)
{
    return $model->getKeyName() === last(explode('.', $key))
                && $model->getIncrementing()
                && in_array($model->getKeyType(), ['int', 'integer'])
                    ? 'whereIntegerInRaw' // <- the method name optimized for integers
                    : 'whereIn';
}

My proposal is to remove the check to $model->getIncrementing() and use the optimization also for integer keys that are not marked for auto incrementation.
As this is a breaking change it should be done for the next big release (5.9 or 6.0) and thus also needs to be documented in the upgrade guide.

[driesvints]

Merged