minor #5641 DX: use constants for PHPUnit version on 3.0 line (keradus)

This PR was merged into the 3.0 branch.

Discussion
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DX: use constants for PHPUnit version on 3.0 line

follows #5640

Commits
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6570f5b DX: use constants for PHPUnit version on 3.0 line

src/RuleSet/Sets/PHPUnit50MigrationRiskySet.php

@@ -14,6 +14,7 @@
 
 namespace PhpCsFixer\RuleSet\Sets;
 
+use PhpCsFixer\Fixer\PhpUnit\PhpUnitTargetVersion;
 use PhpCsFixer\RuleSet\AbstractRuleSetDescription;
 
 /**
@@ -26,7 +27,7 @@ public function getRules(): array
         return [
             '@PHPUnit48Migration:risky' => true,
             'php_unit_dedicate_assert' => [
-                'target' => '5.0',
+                'target' => PhpUnitTargetVersion::VERSION_5_0,
             ],
         ];
     }