minor symfony#26110 Bump default PHPUnit version from 6.3 to 6.5 (derrabus)

This PR was merged into the 3.4 branch.

Discussion
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Bump default PHPUnit version from 6.3 to 6.5

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no, but unrelated
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

This PR bumps the default PHPUnit version for php ≥ 7.2 from the outdated 6.3 to the currently maintained version 6.5.

Commits
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aeffc5f Bump default PHPUnit version from 6.3 to 6.5

Author: nicolas-grekas

src/Symfony/Bridge/PhpUnit/bin/simple-phpunit

@@ -17,7 +17,7 @@ error_reporting(-1);
 
 if (PHP_VERSION_ID >= 70200) {
     // PHPUnit 6 is required for PHP 7.2+
-    $PHPUNIT_VERSION = getenv('SYMFONY_PHPUNIT_VERSION') ?: '6.3';
+    $PHPUNIT_VERSION = getenv('SYMFONY_PHPUNIT_VERSION') ?: '6.5';
 } elseif (PHP_VERSION_ID >= 50600) {
     // PHPUnit 4 does not support PHP 7
     $PHPUNIT_VERSION = getenv('SYMFONY_PHPUNIT_VERSION') ?: '5.7';