Update CHANGELOG with recent changes

The CHANGELOG has been updated to reflect recent changes including dependency upgrades, removal of the PHPStan tool, and transitioning all test classes to use attributes instead of annotations. This update aims to provide a detailed record of modifications for easier project tracking and version control.

Signed-off-by: Marcel Strahl <info@marcel-strahl.de>

CHANGELOG.md

@@ -10,3 +10,7 @@ This Changelog refers to all changes since v5.0.0
 * Vat and Price entity now has a private constructor and is initialized by a new `static` create method.
 * The `ConverterFactory` class uses `match` instead of `switch`.
 * Fixed unwanted forced order in the VAT calculation (for more details see [Upgrade to 5.0 Guide](UPGRADE-5.0.md))
+
+# v5.x.x
++ Upgrade of dependencies, removal of PHPStan as I focused more on Psalm.
++ All test classes are now final and use attributes instead of annotations.