github: use new issue template feature

This helps us to avoid getting feature requests as well as security
bugs reported into the issue tracker.

Closes #5936

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,28 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- Only file bugs here! Ask questions on the mailing lists https://curl.haxx.se/mail/
+
+     SECURITY RELATED? Post it here: https://hackerone.com/curl
+
+     There are collections of known issues to be aware of:
+     https://curl.haxx.se/docs/knownbugs.html
+     https://curl.haxx.se/docs/todo.html       -->
+
+### I did this
+
+### I expected the following
+
+### curl/libcurl version
+
+[curl -V output]
+
+### operating system
+
+<!-- On Unix please post the output of "uname -a" -->