Add kind/deprecation to pull request template

With the merge of kubernetes/test-infra#15040,
we're now able to use the kind/deprecation label for PRs as well. This
label will be consumed by the release notes generation tool.

Signed-off-by: Sascha Grunert <[email protected]>

Author: saschagrunert

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,6 +14,7 @@ https://git.k8s.io/community/contributors/devel/sig-release/release.md#issuepr-k
 > /kind api-change
 > /kind bug
 > /kind cleanup
+> /kind deprecation
 > /kind design
 > /kind documentation
 > /kind failing-test