minor #23766 Consistently use 7 chars of sha256 for hash-based id generation (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
----------

Consistently use 7 chars of sha256 for hash-based id generation

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This prevents generating over long service ids, and for filesystem-related changes, makes the Windows 258 chars limit farther.

Commits
-------

bc22cdd034 Consistently use 7 chars of sha256 for hash-based id generation

Author: fabpot

Field/FileFormField.php

@@ -59,7 +59,7 @@ public function setValue($value)
             $name = $info['basename'];
 
             // copy to a tmp location
-            $tmp = sys_get_temp_dir().'/'.sha1(uniqid(mt_rand(), true));
+            $tmp = sys_get_temp_dir().'/'.strtr(substr(base64_encode(hash('sha256', uniqid(mt_rand(), true), true)), 0, 7), '/', '_');
             if (array_key_exists('extension', $info)) {
                 $tmp .= '.'.$info['extension'];
             }