[Process] Update cherry-pick template assignees (#125402)

Updates the cherry-pick template to add @itsjustkevin as primary and
@CaseyHillers as secondary assignees to all cherry-pick requests.

.github/ISSUE_TEMPLATE/7_cherry_pick.yml

@@ -2,6 +2,9 @@ name: Request a cherry-pick.
 description: As a contributor, you would like to request that a feature be cherry-picked into a release.
 title: '[CP] <title>'
 labels: ['cp: review']
+assignees:
+  - itsjustkevin
+  - caseyhillers
 body:
   - type: input
     id: issue_link