Add mention of Flow to issue template (#10168)

.github/ISSUE_TEMPLATE.md

@@ -1,7 +1,9 @@
 Only use GitHub issues for bugs and feature requests.   
 For general support from the community, see https://vaadin.com/forum
 
+NOTE: Issues for Vaadin Flow (Vaadin v10 Java Framework) should go to https://github.com/vaadin/flow
+
 For bugs, please provide at minimum the following information:
-- Vaadin version
+- Vaadin Framework version (e.g. 8.1.3)
 - Description of the bug
 - If possible, minimal reproducible example