Enable no_useless_concat_operator

[paulbalandan]

Description
Depends on #6674

$ vendor/bin/php-cs-fixer describe no_useless_concat_operator
Description of no_useless_concat_operator rule.
There should not be useless concat operations.

Fixer is configurable using following option:
* juggle_simple_strings (bool): allow for simple string quote juggling if it results in more concat-operations merges; defaults to false

Fixing examples:
 * Example #1. Fixing with the default configuration.
   ---------- begin diff ----------
   --- Original
   +++ New
   @@ -1,2 +1,2 @@
    <?php
   -$a = 'a'.'b';
   +$a = 'ab';

   ----------- end diff -----------

 * Example #2. Fixing with configuration: ['juggle_simple_strings' => true].
   ---------- begin diff ----------
   --- Original
   +++ New
   @@ -1,2 +1,2 @@
    <?php
   -$a = 'a'."b";
   +$a = "ab";

   ----------- end diff -----------

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Please rebase.

[MGatner]

Actually got some changes out of this one! Nice rule.

[MGatner]

🤔

[MGatner]

This one almost got me 😆