Update GeneratedFiles.php

The functionality around the regenerate flag (file `var/.regenerate`) seems broken for some time already. The flag is meant to wipe out generated code (from the `generated/` folder). However, this does not much.

To test for this:
```bash
touch var/.regenerate
mkdir -p generated/code/
touch generated/code/test
```

Expected behaviour: A lock file `var/.regenerate.lock` is generated, the generated code (and specifically, the dummy file `generated/code/test`) is removed and the lock is removed as well, simply by reloading the Magento application (a refresh on the frontend, a call upon `bin/magento`, or something similar) while in the Developer Mode.

Actual behaviour: The `var/.regenerate` just sits there, doing nothing. Likewise, the dummy file `generated/code/test` is still there.

The reason for this is quite simple: Within the main class `\Magento\Framework\Code\GeneratedFiles`, the flag is defined via a constant https://github.com/magento/magento2/blob/2.4-develop/lib/internal/Magento/Framework/Code/GeneratedFiles.php#L24 with a value `/var/.regenerate` (an absolute path) and not `var/.regenerate` (a relative path). Because of this, the write call `isExists()` fails validation https://github.com/magento/magento2/blob/2.4-develop/lib/internal/Magento/Framework/Code/GeneratedFiles.php#L123 and therefore the flag is never picked up upon.

Changing the constant value to `var/.regenerate` makes sure (on my side) that the generated files are cleaned up properly.

Author: jissereitsma

lib/internal/Magento/Framework/Code/GeneratedFiles.php

@@ -21,7 +21,7 @@ class GeneratedFiles
     /**
      * Regenerate flag file name
      */
-    const REGENERATE_FLAG = '/var/.regenerate';
+    const REGENERATE_FLAG = 'var/.regenerate';
 
     /**
      * Regenerate lock file name