Update security policy and issue template

Because Spring Framework already has a security policy, this shows up in
the issue template automatically. This commit removes the extra external
link and updates the original security policy.

See gh-33711

.github/ISSUE_TEMPLATE/config.yml

@@ -1,8 +1,5 @@
 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.