[Mailer] [Amazon] Support for custom headers in ses+api

Updated the CHANGELOG to reflect the addition of custom headers support in the Amazon SES API mailer. This change will be available starting from version 7.3 instead of 7.2 as per symfony#58761.

Author: Spomky

src/Symfony/Component/Mailer/Bridge/Amazon/CHANGELOG.md

@@ -1,7 +1,7 @@
 CHANGELOG
 =========
 
-7.2
+7.3
 ---
 
 * Add support for custom headers in ses+api