Update PR template (rust-lang#2791)

Change the template to be more concise to address the team's concern that the template was too verbose.

The new template is also cleaner since we are now using the PR description as the base for the commit message.
---------
Co-authored-by: Michael Tautschnig <[email protected]>
Co-authored-by: Zyad Hassan <[email protected]>

Author: 3 people

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,35 +1,11 @@
-
-### Description of changes: 
-
-Describe Kani's current behavior and how your code changes that behavior. If there are no issues this PR is resolving, explain why this change is necessary.
-
-### Resolved issues:
+> Please ensure your PR description includes the following:
+> 1. A description of how your changes improve Kani.
+> 2. Some context on the problem you are solving.
+> 3. A list of issues that are resolved by this PR.
+> 4. If you had to perform any manual test, please describe them.
+>
+> **Make sure you remove this list from the final PR description.**
 
 Resolves #ISSUE-NUMBER
 
-### Related RFC:
-
-<!--
-Link to the Tracking RFC issue if this work implements part of an RFC.
--->
-Optional #ISSUE-NUMBER.
-
-### Call-outs:
-
-<!-- 
-Address any potentially confusing code. Is there code added that needs to be cleaned up later? Is there code that is missing because it’s still in development? 
--->
-
-### Testing:
-
-* How is this change tested?
-
-* Is this a refactor change?
-
-### Checklist
-- [ ] Each commit message has a non-empty body, explaining why the change was made
-- [ ] Methods or procedures are documented
-- [ ] Regression or unit tests are included, or existing tests cover the modified code
-- [ ] My PR is restricted to a single feature or bugfix
-
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.