Add GitHub pull request template.

This just provides some sane reminders and ticks a box on the GitHub UI. This
change also cleans up the issue template, as there is already an automatic
link to the repository's security disclosure policy.

PiperOrigin-RevId: 307868833

.github/ISSUE_TEMPLATE/config.yml

@@ -8,7 +8,4 @@ contact_links:
     about: Please see our documentation for debugging tips.
   - name: gVisor User Forum
     url: https://groups.google.com/g/gvisor-users
-    about: Please ask and answer questions here.
-  - name: gVisor Security List
-    url: https://github.com/google/gvisor/blob/master/SECURITY.md
-    about: Please report security vulnerabilities using the process described here.
+    about: Ask and answer general questions here.

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+* [ ] Have you followed the guidelines in [CONTRIBUTING.md](../blob/master/CONTRIBUTING.md)?
+* [ ] Have you formatted and linted your code?
+* [ ] Have you added relevant tests?
+* [ ] Have you added appropriate Fixes & Updates references?
+* [ ] If yes, please erase all these lines!