minor #29175 Bump phpunit XSD version to 5.2 (greg0ire)

This PR was merged into the 2.8 branch.

Discussion
----------

Bump phpunit XSD version to 5.2

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Some attributes being used in the phpunit configuration files, namely
`failOnRisky` and `failOnWarning` were introduced in phpunit 5.2.0. The
Composer configuration shows that tests should run with old versions of
phpunit, but phpunit only validates the configuration against the XSD
since phpunit 7.2.0
These changes can be tested as follows:

```
wget http://schema.phpunit.de/5.2/phpunit.xsd
xargs xmllint --schema phpunit.xsd  1>/dev/null
find src -name phpunit.xml.dist| xargs xmllint --schema phpunit.xsd  1>/dev/null
```

See sebastianbergmann/phpunit@7e06a82
See https://github.com/symfony/symfony/blob/46e3745a03e199e64cc0fcf3284a96b5a25dcee9/composer.json#L98

Commits
-------

4dce4b7c30 Bump phpunit XSD version to 5.2

Author: nicolas-grekas

phpunit.xml.dist

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/4.1/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/5.2/phpunit.xsd"
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"