PHPUnit: update configuration

PHPUnit just released version 9.5.10 and 8.5.21.

This contains a particular (IMO breaking) change:

> * PHPUnit no longer converts PHP deprecations to exceptions by default (configure `convertDeprecationsToExceptions="true"` to enable this)

Let's unpack this:

Previously (PHPUnit < 9.5.10/8.5.21), if PHPUnit would encounter a PHP native deprecation notice, it would:
1. Show a test which causes a deprecation notice to be thrown as **"errored"**,
2. Show the **first** deprecation notice it encountered and
3. PHPUnit would exit with a **non-0 exit code** (2), which will fail a CI build.

As of PHPUnit 9.5.10/8.5.21, if PHPUnit encounters a PHP native deprecation notice, it will no longer do so. Instead PHPUnit will:
1. Show a test which causes a PHP deprecation notice to be thrown as **"risky"**,
2. Show the **all** deprecation notices it encountered and
3. PHPUnit will exit with a **0 exit code**, which will show a CI build as passing.

This commit reverts PHPUnit to the previous behaviour by adding `convertDeprecationsToExceptions="true"` to the PHPUnit configuration.

Refs:
* https://github.com/sebastianbergmann/phpunit/blob/9.5/ChangeLog-8.5.md
* https://github.com/sebastianbergmann/phpunit/blob/9.5/ChangeLog-9.5.md

phpunit.xml.dist

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" backupGlobals="true" backupStaticAttributes="false" bootstrap="phpunit-autoload.php" colors="true" convertErrorsToExceptions="true" convertNoticesToExceptions="true" convertWarningsToExceptions="true" failOnRisky="true" failOnWarning="true" processIsolation="false" stopOnError="false" stopOnFailure="false" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" backupGlobals="true" backupStaticAttributes="false" bootstrap="phpunit-autoload.php" colors="true" convertDeprecationsToExceptions="true" convertErrorsToExceptions="true" convertNoticesToExceptions="true" convertWarningsToExceptions="true" failOnRisky="true" failOnWarning="true" processIsolation="false" stopOnError="false" stopOnFailure="false" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
   <coverage processUncoveredFiles="true">
     <include>
       <directory suffix=".php">./lib</directory>