minor #40792 [Security] Stop using a shared changelog for our security packages (Nyholm)

This PR was squashed before being merged into the 5.3-dev branch.

Discussion
----------

[Security] Stop using a shared changelog for our security packages

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

I understand there are historical reasons for why our four security packages share a changelog. However, I dont believe it makes much sense moving forward.

I suggest that ~~6.0~~ will start using separate changelogs.

#### Update

Lets start in 5.4 for the reasons explained by Christophe

Commits
-------

0b1103ae48 [Security] Stop using a shared changelog for our security packages

Author: wouterj

CHANGELOG.md

@@ -0,0 +1,7 @@
+CHANGELOG
+=========
+
+5.3
+---
+
+The CHANGELOG for version 5.3 and earlier can be found at https://github.com/symfony/symfony/blob/5.3/src/Symfony/Component/Security/CHANGELOG.md