minor #58094 Use Stringable whenever possible (fabpot)

This PR was merged into the 7.2 branch.

Discussion
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Use Stringable whenever possible

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | n/a
| License       | MIT

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Commits
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06b49968a5 Use Stringable whenever possible

Author: fabpot

Generator/UrlGenerator.php

@@ -266,7 +266,7 @@ protected function doGenerate(array $variables, array $defaults, array $requirem
                 if ($vars = get_object_vars($v)) {
                     array_walk_recursive($vars, $caster);
                     $v = $vars;
-                } elseif (method_exists($v, '__toString')) {
+                } elseif ($v instanceof \Stringable) {
                     $v = (string) $v;
                 }
             }