minor #48793 Leverage arrow function syntax for closure (tigitz)

This PR was merged into the 6.3 branch.

Discussion
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Leverage arrow function syntax for closure

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #47658 <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
| License       | MIT
| Doc PR        | <!-- required for new features -->

Rationale in the RFC [here](https://wiki.php.net/rfc/arrow_functions_v2#introduction)

It's also notable that using arrow function syntax rather than the classic one has been enforced in the past by symfony core member: symfony/symfony#48069 (comment)

So this PR would be consistent.

Commits
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f5802d3a2a Leverage arrow function syntax for closure

Author: nicolas-grekas

Filesystem.php

@@ -438,11 +438,9 @@ public function makePathRelative(string $endPath, string $startPath): string
             $startPath = str_replace('\\', '/', $startPath);
         }
 
-        $splitDriveLetter = function ($path) {
-            return (\strlen($path) > 2 && ':' === $path[1] && '/' === $path[2] && ctype_alpha($path[0]))
-                ? [substr($path, 2), strtoupper($path[0])]
-                : [$path, null];
-        };
+        $splitDriveLetter = fn ($path) => (\strlen($path) > 2 && ':' === $path[1] && '/' === $path[2] && ctype_alpha($path[0]))
+            ? [substr($path, 2), strtoupper($path[0])]
+            : [$path, null];
 
         $splitPath = function ($path) {
             $result = [];