chore: add backport to `next` (Vue 3) in the PR checklist

Signed-off-by: Grigorii K. Shartsev <me@shgk.me>

.github/pull_request_template.md

@@ -1,6 +1,6 @@
 ### ☑️ Resolves
 
-* Fix #…
+- Fix #…
 
 ### 🖼️ Screenshots
 
@@ -8,7 +8,6 @@
 ---|---
 B | A
 
-
 ### 🚧 Tasks
 
 - [ ] ...
@@ -17,3 +16,4 @@ B | A
 
 - [ ] ⛑️ Tests are included or are not applicable
 - [ ] 📘 Component documentation has been extended, updated or is not applicable
+- [ ] 3️⃣ Backport to `next` requested with a Vue 3 upgrade