minor #19660 Update version in build.php (AurelienPillevesse)

This PR was merged into the 6.4 branch.

Discussion
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Update version in build.php

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Commits
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e091091 Update build.php

Author: javiereguiluz

_build/build.php

@@ -20,7 +20,7 @@
 
         $outputDir = __DIR__.'/output';
         $buildConfig = (new BuildConfig())
-            ->setSymfonyVersion('5.4')
+            ->setSymfonyVersion('6.4')
             ->setContentDir(__DIR__.'/..')
             ->setOutputDir($outputDir)
             ->setImagesDir(__DIR__.'/output/_images')