Build/Test Tools: Update PHPUnit configuration for PHPUnit 9.5.10/8.5.21+.

Since PHPUnit 9.5.10 and 8.5.21, PHP deprecations are no longer converted to exceptions by default (`convertDeprecationsToExceptions="true"` can be configured to enable this).

Reference: [fac02620f6 Do not convert PHP deprecations to exceptions by default]; [https://github.com/sebastianbergmann/phpunit/blob/9.5.10/ChangeLog-9.5.md#9510---2021-09-25 PHPUnit 9.5.10 changelog].

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. It also adds the other related directives for consistency.

Props jrf, netweb, costdev, SergeyBiryukov.
See #54183.
Built from https://develop.svn.wordpress.org/trunk@51871


git-svn-id: http://core.svn.wordpress.org/trunk@51464 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51870';
+$wp_version = '5.9-alpha-51871';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.