ci: update issue template after Neovim 0.12.0 release

Author: echasnovski

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -76,8 +76,9 @@ body:
         - 0.9.x
         - 0.10.x
         - 0.11.x
-        - 0.12 (!at least latest Nightly build!)
-      default: 2
+        - 0.12.x
+        - 0.13 (!at least latest Nightly build!)
+      default: 3
     validations:
       required: true
   - type: textarea