minor #12542 Change the default mapping type to XML (sdudziak)

This PR was squashed before being merged into the 4.3 branch (closes #12542).

Discussion
----------

Change the default mapping type to XML

YAML mapping are  marked as deprecated for doctrine v2.6+ and will be removed in doctrine v3+ . https://www.doctrine-project.org/projects/doctrine-orm/en/2.6/reference/yaml-mapping.html#yaml-mapping

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

ca4a8a2 Change the default mapping type to XML

Author: xabbuh

doctrine/reverse_engineering.rst

@@ -64,7 +64,7 @@ files: ``BlogPost.php`` and ``BlogComment.php``.
 
 .. tip::
 
-    It's also possible to generate the metadata files into XML or YAML:
+    It's also possible to generate the metadata files into XML or eventually into YAML:
 
     .. code-block:: terminal
 
@@ -82,7 +82,7 @@ files: ``BlogPost.php`` and ``BlogComment.php``.
                 mappings:
                     App:
                         is_bundle: false
-                        type: yml # Set to xml in case of XML mapping
+                        type: xml # "yml" is marked as deprecated for doctrine v2.6+ and will be removed in v3
                         dir: '%kernel.project_dir%/config/doctrine'
                         prefix: 'App\Entity'
                         alias: App