Update CHANGELOG-VAADIN8.md (#12539)

CHANGELOG-VAADIN8.md

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 # Vaadin 8 extended maintenance version changelog
 
+## Vaadin 8.15.2
+
+* Added support for Liferay kernel versions up to 49
+* Changed all resources to use Object.class as interface type to support OSGi Portlets on Liferay CE 7.3.6 GA7 or later (#12504)
+
 ## Vaadin 8.15.1
 
 * Fixed an issue where Grid was moving focus away from external input controls when the datasource contents were updated