minor #28880 [HttpFoundation] Publicify new consts (ro0NL)

This PR was merged into the 4.2-dev branch.

Discussion
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[HttpFoundation] Publicify new consts

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #...   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Continuation of #28807

I think it's reasonable to enforce visibility for new consts, ideally solved by PHP-CS-Fixer. This makes it explicit for the consts to be consumed as part of public API.

Commits
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ce95d0d [HttpFoundation] Publicify new consts

src/Symfony/Component/HttpFoundation/HeaderUtils.php

@@ -18,8 +18,8 @@
  */
 class HeaderUtils
 {
-    const DISPOSITION_ATTACHMENT = 'attachment';
-    const DISPOSITION_INLINE = 'inline';
+    public const DISPOSITION_ATTACHMENT = 'attachment';
+    public const DISPOSITION_INLINE = 'inline';
 
     /**
      * This class should not be instantiated.