feat: use new pull request template (#999)

#### Motivation

Update the Pull Request template to a simpler newer one (using [the
template from the Argo Workflows GitHub
Repository](https://github.com/argoproj/argo-workflows/blob/main/.github/pull_request_template.md)).

#### Modification

Update the PR template.

.github/pull_request_template.md

@@ -1,15 +1,13 @@
-#### Motivation
+### Motivation
 
-_What does this change aim to achieve?_
+<!-- TODO: Say why you made your changes. -->
 
-#### Modification
+### Modifications
 
-_Why is this change being made? What implications or other considerations are there?_
+<!-- TODO: Say what changes you made. -->
 
-#### Checklist
+<!-- TODO: Attach screenshots if you changed the UI. -->
 
-_If not applicable, provide explanation of why._
+### Verification
 
-- [ ] Tests updated
-- [ ] Docs updated
-- [ ] Issue linked in Title
+<!-- TODO: Say how you tested your changes. -->