Revisit GitHub issue template

Prior to this commit, this repository had a single issue template
markdown file that listed all the information for creating a new issue
in this project. Because there was a lot, people missed warnings against
creating security reports publicly or asking questions in the issue
tracker.

This commit introduces an issue template configuration that provides
developers with a choice before creating the actual issue.

Closes gh-33711

Author: bclozel

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,9 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security issue
+    url: https://github.com/spring-projects/security-advisories/security/advisories/new
+    about: Security issues must be disclosed and discussed in private. See https://spring.io/security-policy
+  - name: Asking for help
+    url: https://stackoverflow.com/tags/spring
+    about: The Spring team is using StackOverflow for questions.
+

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,23 @@
+---
+name: Issue
+about: Bugs, enhancements, documentation, tasks.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!--
+
+Have you considered asking for help on stackoverflow.com?
+
+** Bug Reports **
+Please submit issues against OSS supported versions, see https://spring.io/projects/spring-framework#support
+Please provide a minimal sample application that reproduces the problem for faster issue triage.
+
+** Enhancements requests **
+Before explaining how you would like things to work,
+please describe a concrete use case for this feature and how you have tried to solve this so far.
+
+-->
+