minor #25653 PHP CS Fixer: clean up repo and adjust config (keradus)

This PR was squashed before being merged into the 2.7 branch (closes #25653).

Discussion
----------

PHP CS Fixer: clean up repo and adjust config

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Reason for this PR is that one want to have `php-cs-fixer fix -v` command executed without changes that shall not be applied for this repo. To achieve that, we need to groom config to exclude files that violate CS willingly, fix files that are violating CS unwillingly, and deliver missing case handling at PHP CS Fixer itself (PHP-CS-Fixer/PHP-CS-Fixer#3359) (already merged!).

Commits
-------

b14cbc1 PHP CS Fixer: clean up repo and adjust config

Author: nicolas-grekas

Session/Storage/Handler/LegacyPdoSessionHandler.php

@@ -54,6 +54,8 @@ class LegacyPdoSessionHandler implements \SessionHandlerInterface
     private $timeCol;
 
     /**
+     * Constructor.
+     *
      * List of available options:
      *  * db_table: The name of the table [required]
      *  * db_id_col: The column where to store the session id [default: sess_id]

Session/Storage/Handler/MemcacheSessionHandler.php

@@ -29,6 +29,8 @@ class MemcacheSessionHandler implements \SessionHandlerInterface
     private $prefix;
 
     /**
+     * Constructor.
+     *
      * List of available options:
      *  * prefix: The prefix to use for the memcache keys in order to avoid collision
      *  * expiretime: The time to live in seconds

Session/Storage/Handler/MemcachedSessionHandler.php

@@ -34,6 +34,8 @@ class MemcachedSessionHandler implements \SessionHandlerInterface
     private $prefix;
 
     /**
+     * Constructor.
+     *
      * List of available options:
      *  * prefix: The prefix to use for the memcached keys in order to avoid collision
      *  * expiretime: The time to live in seconds

Session/Storage/Handler/MongoDbSessionHandler.php

@@ -29,6 +29,8 @@ class MongoDbSessionHandler implements \SessionHandlerInterface
     private $options;
 
     /**
+     * Constructor.
+     *
      * List of available options:
      *  * database: The name of the database [required]
      *  * collection: The name of the collection [required]