Introduce issue template (#8043)

Introduce issue template to raise awareness of forums and the quality of bug reports.

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+Only use GitHub issues for bugs and feature requests.   
+For general support from the community, see https://vaadin.com/forum
+
+For bugs, please provide at minimum the following information:
+- Vaadin version
+- Description of the bug
+- If possible, minimal reproducible example