Make HasTimestamps::updateTimestamps public (#33953)

I've a use-case where I construct models but can't save them yet but
they should reflect the state of new models as good as it gets,
including the timestamps for created/updated, which I want to do from
outside the model

Similar to the `touch()` method:
```php
    public function touch()
    {
        if (! $this->usesTimestamps()) {
            return false;
        }

        $this->updateTimestamps();

        return $this->save();
    }
```
But I don't want to save the model _yet_.

I can call `usesTimestamps()` from outside the model as its
already public but not yet `updateTimestamps`, thus the PR.

Author: mfn

src/Illuminate/Database/Eloquent/Concerns/HasTimestamps.php

@@ -34,7 +34,7 @@ public function touch()
      *
      * @return void
      */
-    protected function updateTimestamps()
+    public function updateTimestamps()
     {
         $time = $this->freshTimestamp();