Update phpstan/phpstan-phpunit requirement from ^1.3.11 to ^1.3.15 (#37)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -14,7 +14,7 @@
         "editorconfig-checker/editorconfig-checker": "^10.5.0",
         "ergebnis/composer-normalize": "^2.30.2",
         "phpstan/phpstan": "^1.10.13",
-        "phpstan/phpstan-phpunit": "^1.3.11",
+        "phpstan/phpstan-phpunit": "^1.3.15",
         "phpstan/phpstan-strict-rules": "^1.5.2",
         "phpunit/phpunit": "^10.1",
         "shipmonk/composer-dependency-analyser": "^1.2",