Prefer "exclude" format over "severity 0" format in ruleset for readability

Problem/Motivation

Coming from #2901739: Fix 'Squiz.Arrays.ArrayDeclaration' coding standard. We have two different ways of excluding sub-rules in core's phpcs.xml.dist:

1.     <exclude name="Drupal.Commenting.ClassComment.Missing"/>
   

2.   <rule ref="PEAR.Files.IncludingFile.UseIncludeOnce">
       <severity>0</severity>
     </rule>
   

We want core's phpcs.xml.dist to match the Drupal standard.

Proposed resolution

Prefer the "exclude" format, since it is less verbose and more readable. According to @mfernea there is not a difference between the two for our usecase.

Reference: https://github.com/squizlabs/PHP_CodeSniffer/wiki/Annotated-ruleset.xml

Update the ruleset to that end: http://cgit.drupalcode.org/coder/tree/coder_sniffer/Drupal/ruleset.xml

Remaining tasks

Once we've updated the coder standards, update core's phpcs.xml.dist to match: #2909268: Prefer "exclude" format over "severity 0" format in ruleset for readability once coder does

mfernea should receive credit for this issue.