PHPORM-264: Deprecate Mongo soft deletes trait

[paulinevos]

In favor of the Laravel one. Ours is now obsolete; the SoftDeletes trait is only necessary to remove the call to qualifyColumn in the parent trait. But the DocumentModel::qualifyColumn is already disabled.

Also replaces some internal/test usages with the recommended trait.

Closes PHPORM-264

Checklist

• Ensure tests still pass

[rustagir]

changes to docs tests lgtm

[GromNaN]

We can trigger a deprecation when the PHP file is loaded.

In the @deprecated annotation, we usually give the version of the deprecation.

Suggested change

	/** @deprecated 6.0.0 in favor of \Illuminate\Database\Eloquent\SoftDeletes */
	trigger_error(sprintf('Since mongodb/laravel-mongodb:5.5, trait "%s" is deprecated, use "%s" instead.', SoftDeletes::class, \Illuminate\Database\Eloquent\SoftDeletes::class), E_USER_DEPRECATED);
	/** @deprecated since mongodb/laravel-mongodb:5.5, use \Illuminate\Database\Eloquent\SoftDeletes instead */

Example of an other deprecation I put in the library:

laravel-mongodb/src/Connection.php

Lines 126 to 132 in b02b40c

	* @deprecated since mongodb/laravel-mongodb:5.2, use getDatabase() instead
	*
	* @return Database
	*/
	public function getMongoDB()
	{
	trigger_error(sprintf('Since mongodb/laravel-mongodb:5.2, Method "%s()" is deprecated, use "getDatabase()" instead.', __FUNCTION__), E_USER_DEPRECATED);

An other example of deprecating a class in Symfony: https://github.com/symfony/symfony/blob/138109a63e2202674d81f31481da7c6defe4c0ce/src/Symfony/Component/Serializer/CacheWarmer/CompiledClassMetadataCacheWarmer.php#L19