Resolve incorrect violation code in `Magento2.Security.XssTemplate` sniff

[aadmathijssen]

This PR resolves the issue that in the Magento2.Security.XssTemplate sniff FoundUnescaped violations are sometimes incorrectly reported as FoundNotAllowed.

This currently happens from the moment the first FoundNotAllowed is reported, after that all violations are reported as FoundNotAllowed.

Consider the following code:

<?= /* @disallowedAnnotation */ $foo ?>
<?= $bar ?>

The following is reported (the second warning has the wrong code):

FILE: /path/to/file.phtml
------------------------------------------------------------------------------------------
FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
------------------------------------------------------------------------------------------
 1 | WARNING | Unescaped output detected.
   |         | (Magento2.Security.XssTemplate.FoundNotAllowed)
 2 | WARNING | Unescaped output detected.
   |         | (Magento2.Security.XssTemplate.FoundNotAllowed)
------------------------------------------------------------------------------------------

This PR makes sure the following is reported:

FILE: /path/to/file.phtml
------------------------------------------------------------------------------------------
FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
------------------------------------------------------------------------------------------
 1 | WARNING | Unescaped output detected.
   |         | (Magento2.Security.XssTemplate.FoundNotAllowed)
 2 | WARNING | Unescaped output detected.
   |         | (Magento2.Security.XssTemplate.FoundUnescaped)
------------------------------------------------------------------------------------------

This is done by resetting the hasDisallowedAnnotation property at then end of processing.