Add PR link to changelog

Author: subhramit

CHANGELOG.md

@@ -17,7 +17,7 @@ Note that this project **does not** adhere to [Semantic Versioning](https://semv
 
 ### Removed
 
-- We removed support for MySQL/MariaDB and Oracle.
+- We removed support for MySQL/MariaDB and Oracle. [#12990](https://github.com/JabRef/jabref/pull/12990)
 - We removed library migrations (users need to use JabRef 6.0-alpha.1 to perform migrations) [#12990](https://github.com/JabRef/jabref/pull/12990)
 
 ## [6.0-alpha2] – 2025-04-27