ci: Skip flaky test on PHP 8.3

Signed-off-by: Joas Schilling <coding@schilljs.com>

tests/lib/Files/ObjectStore/S3Test.php

@@ -134,6 +134,10 @@ class S3Test extends ObjectStoreTest {
 
 	/** @dataProvider dataFileSizes */
 	public function testFileSizes($size) {
+		if (str_starts_with(PHP_VERSION, '8.3') && getenv('CI')) {
+			$this->markTestSkipped('Test is unreliable and skipped on 8.3');
+		}
+
 		$this->cleanupAfter('testfilesizes');
 		$s3 = $this->getInstance();